Ube "Westminster" Scries 



SOILS' AND MANURES 



IN THE SAME SERIES 

COAL. By JAMES TONGE, 
M.I.M.B., F.G.S., &c., Lecturer 
on Mining at A'ictoria Univer- 
sity, Manchester. 

NATURAL SOURCES OF 
POWER. By ROBERT S. 
BALL, B.Sc., A.M.I.C.TC. 

PATENTS, TRADE MARKS, 
AND DESIGNS. By KEN- 
NETH R. SWAN, B.A. (Oxon). 
of the Inner Temple. Barrister- 
at-Law. 

TIMBER. By J. R. BATERDEN, 
A.M.I.C.E. 

THE CHEMISTRY OF 
BUILDING MATERIALS. 

By ALAN MUNBY. A. R.I. B.A. 
ETC. 

See complete jmmjtectiis. 



SOILS AND MANURES 



BY 



J. ALAN MURRAY, B.Sc. 





NEW YORK 

D. VAN NOSTRAND COMPANY 

23 MURRAY AND 27 WARREN STREETS 
igio 



PREFACE 

THE present work is intended mainly for those who are, 
or who expect to become, interested in the cultivation of 
land. The author has a large acquaintance amongst men 
of that class farmers, planters, fruit growers, etc. and 
has himself taken part in the practical work, both in this 
country and in Canada, and he believes that it is in regard 
to the fundamental truths which underlie the phenomena 
of agriculture that they stand most in need of instruction. 
As a rule the mechanical part of the work is organised and 
carried out with systematic efficiency, and when mistakes 
are made they are generally attributable to lack of know- 
ledge of the kind referred to. 

The subject has therefore been treated from what is 
popularly called the scientific point of view. Some previous 
knowledge of chemistry is necessary, and it has been 
assumed that readers are familiar with at least the rudi- 
ments of that science. For purely practical purposes this 
will probably be found sufficient, but it is obvious that 
wider general reading must be of great advantage to the 
student. 

In the chapters on Fertility and the Principles of 
Manuring a number of ideas are collated which could not 
be adequately discussed in either of the two main sections 
(Soils and Manures) of the book between which they form 



of 



vi PREFACE 

connecting links. The statistics of imports and production 
quoted from the official reports of the Board of Agriculture 
and trade journals, and also accounts of various substances 
e.g., raw phosphates, ground leather, which the ordinary 
farmer rarely meets with in that form are perhaps of 
greater interest from a commercial than a strictly agricul- 
tural standpoint. They have, however, a certain import- 
ance and may be considered necessary for completeness in 
a work of this character. The descriptions of technical 
processes by which basic slag and some other substances 
are obtained are intended merely to illustrate the nature of 
the products and are necessarily much condensed. The 
prices mentioned in discussing the relative pecuniary value 
of certain kinds of manure are those prevailing at the time 
of writing but are not of permanent interest. Instructions 
for valuing manures issued by the Highland and Agricul- 
tural Society of Scotland and the estimated manurial value 
of the commoner kinds of feeding stuffs are given in 
appendices. The latter is from a recent paper by Dr. 
Charles Crowther. 

In a few instances analytical methods have been brietly 
outlined as a convenient plan of developing certain ideas, 
but details have been avoided. Many of the figures relating 
to the composition of manures are taken from published 
reports by Dr. Voelcker and others, but some of them are 
average results of numerous analyses made by the author. 

The facts and figures selected to illustrate established 
principles have been chosen, as far as possible, from the 
Bothamsted records, partly on account of their recognised 
reliability and partly, also, because these classic researches 
are likely to prove of greater interest to English students 



PEEFACE vii 

than others which have been carried out abroad. Many 
works of reference, periodicals and other sources have been 
consulted. Particular informations are acknowledged in 
their places ; but the author desires to express his indebted- 
ness to many other writers particularly Dr. Fream, Mr. 
J. E. Warington and Mr. A. D. Hall for much that cannot 
be specifically mentioned. 

Several of the illustrations in the biological section were 
supplied by Mr. John Golding of the Midland Agricultural 
College, and some of the others are due to the courtesy of 
various commercial firms. Thanks are due also to the 
editor of the Journal of Gas Lighting, the secretary of 
the Iron and Steel Institute, Mons. Eoussel of the Institut 
Pasteur, and Mr. W. A. Cox of Albert's, for kind assistance 
in this connection. 

J. M. 

CROSBY, 

1909. 



TABLE OF CONTENTS 



PAGE 

PREFACE v 



CHAPTEK I. 

INTRODUCTORY. 

Vegetation. Soils. Manures. Plants : Composition Sources of con- 
stituents Sand cultures Water cultures. The atmosphere : 
Constituents Oxygen Nitrogen Water vapour Carbon 
dioxide Impurities Bain water p. I 



CHAPTER II. 

THE ORIGIN OF SOILS. 

Minerals : Magnetite Haematite Limonite Pyrites Eluorite 
Calcite Gypsum Apatite Quartz Olivine Mica Leucite 
Augite Hornblende Felspars Nepheline Kaolin Zeolites 
Chemical composition of minerals Composition of the crust 
of the earth. Rocks : Igneous rocks Aqueous rocks Glaciation 
Weathering Denudation Organic rocks Stratification 
Sand Clay Shale Marl Limestone Peat. Summary : 
Formation of soils Classification p. 18 



CHAPTEE III. 

PHYSICAL PROPERTIES OF SOILS. 

The particles : Size Number Arrangement Interspace Internal 
surface Mass. Moisture : Precipitation Hygroscopy Deli- 
quescence .Capacity Retention Capillarity Percolation 



TABLE OF CONTENTS 

Evaporation. Temperature: Influence of latitude Altitude 
Aspect Colour Capacity for heat Eadiation Conduction . 
Miscellaneous properties : Colour Odour Tenacity Contraction 
Diffusion. Mechanical analysis j>. .00 



CHAPTER IV. 

CHEMISTHY OF SOILS. 

Chemical analysis : Soil water Absorption Organic matter Lime 
Insoluble mineral residue Total acid extract Available plant 
foods. Chemical changes: Decomposition of minerals Oxi- 
dation and reduction Dehydration Other reactions Soil 
gases . . . , p. 96 



CHAPTER Y. 

BIOLOGY OF SOILS. 

The organisms. Algse. Moulds. Bacteria Reproduction Appear- 
ance Food Antiseptics Sterilisation Chemical changes 
Fixation of free nitrogen Hellriegel's discovery Sand cultures 
Seat of fixation Inoculation of soils Fixation by non- 
leguminous plants. Destruction of organic matter : Decay and 
putrefaction Composts. Nitrification : Nature of change Pro- 
duction of ammonia Nitrous and nitric ferments Simultaneous 
action Habitat and conditions of growth Rate of nitrification 
Denitrification Practical importance . . . . jt. 137 



CHAPTER VI. 

FERTILITY. 

Natural productiveness. Effects of removing crops. Maintenance of 
fertility : Fallow Lay Restitution Quantity of available plant 
foods required by crops Loss. Addition : Essential difference 
between farmyard manure arid fertilisers. Conservation of 
matter .......... p. 163 



TABLE OF CONTENTS xi 

CHAPTER VII. 

PRINCIPLES OF MANURING. 

Definition of manure. Functions of manures. Adaptation to circum- 
stances. Requirements of soils : Too rich soils Determination 
of requirements Eight-plot test Wire-basket method 
Chemical analysis. Requirements of crops: Chemical composi- 
tion Classification of crops Cereals and grass Roots Potatoes 
Leguminous crops Garden crops. Classification of manures 

p. 178 

CHAPTER VIII. 

PHOSPHATIC MANURES. 

Native phosphates : Use as manure General properties Production 
and imports Apatites and phosphorites Coprolites Phosphatic 
guanos. Superphosphates : Manufacture Composition and value 
Reverted phosphates Manurial action Effects of mixing with 
other manures. Basic slag : Source Composition Basic 
character Manurial effects Compared with superphosphate 
Application Mechanical condition Adulteration Artificial 
basic slags Precipitated phosphate . . . . . p. 206 



CHAPTER IX. 

PHOSPHO-NITROGENOUS MANURES. 

Imports Composition Methods of treatment Crushed 
bones Steamed bones Bone-ash Bone-char Dissolved bones. 
Meat meals. Fish guanos p. 237 

CHAPTER X. 

NITROGENOUS MANURES. 

Organic nitrogenous manures Leather Hoof and horn meals 
Woollen rags and shoddy. Sulphate of ammonia : Production- 
Composition and properties Adulteration Manurial value 



xii TABLE OF CONTENTS 

Application. Nitrate of soda : Occurrence Extraction Com- 
position and properties Adulteration Manurial value Applica- 
tion Compared with sulphate of ammonia Exhaustive effects. 
Other nitrates : Nitrate of lime. Nitrolim : Composition and 
properties Application Experimental results . . p. 249 

CHAPTER XI. 

fOTASH MANURES. 

Sources Potash minerals Wood ashes. Potash salts Occurrence 
Sylvine Sylvinite Carnallite Schonite Kainite Polyhalite 
Kieserite Composition Manurial value Influence of 
chlorides Comparison of sulphate and muriate Soils suitable 
for Crops which require potash manures . . . />. 270 



CHAPTER XII. 

COMPOUND AND MISCELLANEOUS MANURES. 

Compoundmanures Turnip manures Potato manures. Miscellaneous 
manures Common salt : Gypsum Copperas Magnesium 
salts p. 287 

CHAPTER XIII. 

GENERAL MANURES. 

Guano Introduction Artificial Origin and occurrence Preserva- 
tion of deposits Nitrogenous and phosphatic guanos Imports 
Composition Manurial value Application. Sewage Sludge 
manures Liquid manure. Vegetable manures : Green manures 
Seaweed Waste cake Leaf-mould . . . . p. 292 

CHAPTER XIY. 

FARMYARD MANURE. 

The excreta Dung Urine. Litter: Straw Peat moss Dried 
leaves Other litters Quantity of litter. Fermentation Non- 



TABLE OF CONTENTS xiii 

nitrogenous matter Nitrogenous matter. Loss in making and 
storing : Drainage Evaporation. Ammonia fixers : Gypsum 
Antiseptics Absorptives. Composition. Manurial value : 
Mechanical effects Fertilising ingredients State of combination. 
Application p. 305 



APPENDICES. 

I. VALUATION OF MANURES p. 333 

II. COMPOSITION AND MANURIAL VALUE OF VARIOUS FARM 

FOODS . .p. 338 



INDEX p. 341 




SOILS AND MANURES 



CHAPTEE I 

INTRODUCTORY 

UNCULTIVATED land is generally covered with vegetation 
of some kind. The plants grow without assistance in any 
form from man. In fact, one of the difficulties with 
which farmers have to contend is to keep down this natural 
vegetation which interferes with the growth of crops. The 
amount of produce from a given area of land, under purely 
natural conditions, is, however, relatively small, and one 
of the principal objects of cultivation is to increase it. 
Improvements have been effected by experiments on 
the plants themselves, some of which have been greatly 
modified and rendered more suitable for use as food and 
other purposes for which they are grown. This, how- 
ever, is chiefly the work of specialists. The part of the 
ordinary farmer is generally confined to the reproduction 
of established types and assisting the development of the 
plants by improving the conditions under which they grow. 
This is effected chiefly by draining, tilling and manuring 
the land in order to remove injurious substances and pro- 
vide a plentiful supply of air, water and plant foods, i.e., 
substances which the plants contain and which are neces- 
sary for their growth. 

Soil. The soil is the natural habitat of plants. In 
S.M. B 



2 SOILS AND MANUKES 

general, it consists of a heterogeneous mixture of loose 
mineral and organic matter, but the bulk of it is not plant 
food at all. On the contrary, it consists of material which, 
by reason either of its physical condition or its chemical 
nature, is incapable of yielding any nourishment to plants 
growing in it. Natural soils, of course, always contain 
some plant food mixed with the great mass of inert matter 
and, for the most part, derived from it by physical, 
chemical and biological processes. The distinction between 
these different processes is somewhat arbitrary, but it is 
a convenient one for purposes of reference and explana- 
tion, and it has become customary to speak of the 
physical, chemical and biological properties of the soil. 

It is obvious that the insoluble and chemically inert 
matters, of which the soil is mainly composed, must exer- 
cise a preponderating influence upon its physical proper- 
ties. These substances are, therefore, often called the 
physical or mechanical constituents. The soluble and more 
active substances, salts, etc., which are in a condition to 
be taken up by plants, are rarely present in quantity suffi- 
cient to appreciably affect the physical properties, and 
are, by contrast, often referred to as the chemical con- 
stituents. It was at one time generally supposed that the 
productiveness of the soil depends mainly, if not entirely, 
upon the physical properties. In later times the import- 
ance of the chemical constituents became more manifest, 
and perhaps undue emphasis was placed upon it by scien- 
tific writers. Until within comparatively recent years 
nothing was known of the biological properties of soils, 
and that aspect of the subject was entirely ignored. With 
increase of knowledge these things are now seen in more 
just proportion, and it is recognised that a high degree 
of productiveness can only be attained when all the con- 
ditions are complied with. 

A casual inspection of soils reveals marked differences 



INTEODUCTOEY 3 

of colour, texture and other properties, and the differences 
are generally intensified by cultivation. It is well known 
that some soils are naturally much more productive than 
others. They are called rich or poor accordingly. Some 
of them are better adapted for the growth of certain kinds 
of crops, and some are more difficult and expensive to work 
than others. Careful study of the origin and properties of 
soils is necessary in order to arrive at a clear understanding 
of the causes which underlie these differences and the kind 
of treatment which is most appropriate in each case. 

Manure. The expression " manure," as commonly used 
by farmers, is generally understood to refer to the mixture 
of animal dejecta and litter removed from the cowhouses, 
stables and pens in which the animals are confined. From 
time immemorial it has been customary to carry out this 
material to the land and bury it, with the double purpose 
of getting rid of offensive matter and of fertilising the 
soil. Various theories have been propounded, from time 
to time, to account for this fertilising effect of manure, 
but very little definite knowledge was gained until the 
science of chemistry became sufficiently advanced to in- 
vestigate the subject. The truth was then established that 
the manure contains plant foods. In the light of present- 
day knowledge it is easy to see that vegetable matter, of 
which the litter is generally composed, and also animal 
matter derived from it, must of necessity contain sub- 
stances required for the growth of plants. It was soon 
recognised that these same plant foods could also be 
obtained from other sources, and it was thought probable 
that the substances which contain them could also be used 
to fertilise the soil. Experience has amply justified 
this conclusion, and such products are now also called 
manures, or, more commonly, " artificial manures," to 
distinguish them from the farmyard refuse to which alone 
the name had hitherto been applied. 

B 2 



4 SOILS AND MANURES 

THE PLANT. 

Composition of Plants. Plants are composed of certain 
chemical elements. These must be supplied from some- 
where or the plants cannot be formed. They can be 
determined by chemical analysis, and the sources from 
which they are derived can be traced by further experi- 
ment. The methods of analysis of plants involve numerous 
complex processes, a detailed description of which lies 
outside the scope of this work. A brief outline, however, 
is easily followed, and may help to convey some idea of 
the properties and relations of the substances referred to. 

If a plant, or quantity of any vegetable matter, be dried 
as, for example, when grass is made into hay put into 
a crucible and heated to redness, it takes fire and burns. 
The great bulk of it disappears and only a small quantity 
of ash remains behind in the crucible. The part which 
disappears on incineration is commonly spoken of as the 
organic matter or volatile portion, and the ash is, by con- 
trast, known as the mineral or non-volatile matter. These 
names are apt to be misleading ; the difference between 
the two kinds of matter implied by them is not real and 
true. The distinction, however, is a common and con- 
venient one, as the two portions can be examined and con- 
sidered separately. 

The relative proportions of ash and organic matter vary 
in different kinds of plants, in different parts of plants, 
and even in individual specimens. The table on page 5 shows 
the proportions in which they are commonly present. 

The part which is volatilised x when the plant is burned 

1 The organic matter is not volatilised as such. The elements of 
which it is composed except nitrogen, which is liberated in the free 
state unite with the oxygen of the air, forming volatile oxides. The 
oxygen found in the products of combustion is therefore mainly 
derived from the air, but it can be shown that a part of it pre-existed 
in the plant. 



INTKODUCTOKY 



is not destroyed but escapes into the air. By means of 
suitable apparatus it may be collected and examined. It 
will be found to contain the elements hydrogen, oxygen, 
nitrogen, carbon and sulphur. The proportion in which 
these elements are present necessarily varies considerably 
in different circumstances, but roughly, carbon may form 
about half the total weight, oxygen a third, hydrogen a 

AVERAGE PROPORTIONS OF ASH AND ORGANIC MATTER IN THE 
DRY MATTER OF PLANTS. 



: 


Wheat. 


Oats. 


Peas. 


Grain. 


Straw. 


Grain. 


Straw. 


Grain. 


Straw. 


Ash . 
Organic matter 


Per cent. 

2-8 

97-7 


Per cent. 

6-5 
93-5 


Per cent. 

3-2 

96-8 


Per cent. 

5-2 
94-8 


Per cent. 

2-6 
97-4 


Per cent. 

4-9 
95-1 





Potatoes. 


Turnips. 


Meadow 


Beech. 




Tubers. 


Haulm. 


Root. 


Leaf. 




Wood. 


Leaf. 




Per 

cent. 


Per- 
cent. 


Per 

cent. 


Per 

cent. 


Per cent. 


Per 

cent. 


Per 

cent. 


Ash . 


4-3 


5-2 


8-1 


10-9 7-2 


0-9 


5-6 


Organic matter 


95-7 


94-8 


91-9 


89-1 92-8 


99-1 


94-4 



tenth, nitrogen a twentieth, and sulphur less than a hun- 
dredth part of the whole. 

It will be seen, on reference to the table, that the ash 
rarely exceeds about five per cent, of the dry matter in 
stems and leaves. In seeds it is often less than half that 
amount. Nevertheless, the ash is just as important for 
the growth of the plant as the organic matter. It always 
contains the elements potassium, magnesium, calcium, 
iron, phosphorus and usually also sodium, silicon, fluorine 
and chlorine. Traces of lithium, manganese, and some 



SOILS AND MANUEES 



other substances are occasionally present. The chlorine 
and fluorine are always combined with other elements ; 
the remainder occur in the form of oxides. Some of these 
oxides are further combined together in the form of salts, 
but, as it is difficult to determine exactly how the various 
combinations are arranged, they are usually stated separ- 
ately as simple oxides. The proportions of the various 
ingredients vary like that of the total ash. The averages 
in certain typical cases are shown in the following table : 

AVERAGE COMPOSITION OF THE ASH OF PLANTS. 





Barley. 


Beans. 


Turnips. 


Meadow 


Red 




Grain . 


Straw. 


Grain. 


Straw. 


Root. 


Leaf. 


Grass. 


Clover. 




Per 


Per 


Per 


Per 


Per 


Per 


Per 


Per 




cent. 


cent. 


cent. 


cent. 


c^nt. 


cent. 


CPU t. 


cent. 


Potash 


21-2 


21-6 


38-5 


32-7 


47-6 


28-1 


25-6 


32-1 


Soda . 


3-5 


4-1 


6-0 


8-7 


8-7 


6-0 


4-6 


2-0 


Magnesia . 


8-2 


2-4 


7-3 


7-3 


2-6 


2-5 


7-2 


10-8 


Lime . 


2-3 


7-7 


6-3 


25-3 


12-1 


32-8 


16-3 


34-9 


Oxide of iron 


0-3 


0-5 


0-2 


1-7 


0-4 


0-8 


0-8 


0-2 


Phosphoric acid . 82*8 


4-5 


34-6 


7-9 


10-6 


6-7 


6-3 


9-6 


Sulphuric acid . 


2-0 


3-7 


3-2 


2-2 


12-3 


13-3 


3-0 


3-6 


Chlorine 


1-0 


2-6 


1-5 


7-3 


5-1 


8-7 


r r4 


3-8 


Silica . 


28-4 


52-1 


0-8 


5-5 


0-7 


1-5 


28-6 


3-0 



It will be seen that all the products are rich in potash 
and that the grains are rich also in phosphoric acid. The 
turnip leaves, bean straw and clover contain more lime, 
and the barley and meadow grass more silica than the 
others. It has been said that sodium, silicon, chlorine and 
fluorine are commonly present in the ash, but they do not 
appear to be essential for the growth of the plant. They 
may, however, be useful in other ways. For example, the 
outer ^husk of cereals, which forms a protective covering 
to the grain, is largely composed of silica. Chlorine, on 
the other hand,, is not only of no use whatever, but, in large 



INTRODUCTORY 7 

quantities, its effects are highly deleterious to the majority 
of plants. Sodium again is entirely indifferent. Such 
substances are regarded as accidental because they are 
probably taken up by the plants, to a large extent, in 
combination with the other elements, e.g., potassium, 
calcium, etc., which have an active role in the vital pro- 
cesses of the organism. 

Sources of the Constituents. Whatever is found in the 
plant must have been derived at least proximately either 
from the soil or from the air. Sulphur, hydrogen, and the 
elements of the ash are not constituents of the air, and 
must therefore be obtained from the soil. The source of 
the carbon, nitrogen, and oxygen, which occur normally 
both in the air and in the soil, can be determined only by 
experiment. A simple plan, generally applicable to pro- 
blems of this description, is to grow the plants in an arti- 
ficial soil, or medium of some kind to which they are 
entirely indifferent. The effects of various substances can 
then be estimated by introducing them to, or withholding 
them from the medium. 

Sand Cultures. Many different substances have been 
employed as mediums, but, for most purposes, pure 
sand (powdered quartz) is the most suitable. It is cheap, 
easily procured and purified. It is one of the largest con- 
stituents of most ordinary soils, and the plants are, there- 
fore, to that extent, grown under normal conditions. 
If the sand be kept moist and at a suitable temperature; 
seeds sown in it readily germinate and continue to grow 
until the nourishment stored up in them is exhausted. 
Further development cannot take place, and the plants die 
for lack of food, for, it is assumed, there is none in the 
soil. If the experiment be varied by mixing with the sand 
small quantities of the various constituents of plants, 
the seedlings will continue to thrive and grow into 
fully developed plants which may, in time, bear seed. 



8 SOILS AND MANUBES 

By further experiment it can be ascertained that the omis- 
sion of sodium, silicon, chlorine and fluorine compounds 
makes no apparent difference to the growth of the plants, 
and, of course, these elements would not then be found 
in the plants when grown. It is for this reason that they 
are deemed non-essential. Neither does the omission 
of carbon compounds from the mixture retard the growth 
in any way, and, as the usual amount of carbon is found 
in the plants, it must be concluded that this element is 
derived from the air. If, however, nitrogen, sulphur or 
any one of the elements of the ash, other than those men- 
tioned above, be entirely withheld, the result will be the 
same as if none of them at all were present. The plant 
will die when the nourishment stored up in the seed is 
exhausted. It appears, therefore, that nitrogen is derived 
from the soil not from the air l and that nitrogen and 
the other elements, viz., potassium, magnesium, calcium, 
iron, sulphur and phosphorus are absolutely indispensable. 
There is evidence of another kind that they perform impor- 
tant physiological functions. 

It can also be demonstrated by experiments on these 
lines, that if any one of the essential elements be supplied 
in insufficient quantity the growth of the plants will be 
correspondingly limited and cannot be maintained by 
increasing the quantities of any or all of the others. 

Water Cultures. Similar experiments may be con- 
ducted, by another method known as " water culture." The 
principle is the same, but sand is dispensed with and water 
alone is used as a medium or vehicle to convey the nourish- 
ment to the roots of the growing plant. The various sub- 
stances are dissolved in pure distilled water and the solu- 
tion put into a clean wide-mouthed bottle. The seedling 

1 This is a question of the highest importance. There are certain 
exceptions to the general rale stated in the text. The subject is more 
fully discussed at a later stage, 



INTRODUCTORY 9 

is secured between two halves of a cork with a hole in the 
middle. The cork is fitted into the neck of the bottle so that 
the roots are entirely submerged in the liquid and the stem 




FIG. 1. 



erect in the air, as shown in the illustration (Fig. 1). This 
method possesses certain advantages. Sand and every- 
thing else except water, which has to be used in any case 
are entirely dispensed with. The solution is absolutely 
homogeneous throughout. It can be diluted to any extent 



10 SOILS AND MANURES 

and each constituent brought into touch with the roots in 
exactly the proportion desired. On the other hand, the 
experiment cannot be begun with the seed ; the plant must 
be at least partially developed first. Water does not sup- 
port the plant in the upright position, and some arrange- 
ment of corks or wires is necessary for that purpose. 
Access of light to the roots permits the development of 
algse and other growths. The small amount of air which 
can be dissolved in the water is altogether inadequate, and 
when it is exhausted the roots become covered over with the 
products of reduction which are poisonous and seriously 
interfere with the growth of the plants. Of course, all these 
difficulties can be overcome in one way or another and the 
plants can be successfully grown. The plan, however, 
involves so many abnormal conditions that it is scarcely 
applicable except for special purposes. It is quoted here 
chiefly because it serves to illustrate certain important 
points relating to the functions of soils which will be 
emphasised later on. It confirms the conclusions drawn 
from the sand culture experiments, viz., that certain ele- 
ments (vide ante) are indispensable for the growth of 
plants, and that all of them except carbon, which comes 
from the air, are taken up by the roots. It can also be 
shown by experiments of this kind (water cultures and 
sand cultures) that the elements may be supplied to the 
plants in several different forms with equally good results ; 
in fact, if introduced into the soil in almost any form what- 
ever, they will ultimately benefit the plant, but the action 
will be quicker or slower according to the solubility of the 
compounds. 

THE ATMOSPHERE. 

Constituents. The air or atmosphere consists mainly 
of two gases called respectively oxygen and nitrogen. 
They are present in the proportion of one to four, or, 



INTRODUCTORY 11 

more exactly, 21 per cent, of the former and 79 
per cent, of the latter by volume. The gases are not 
chemically united, merely mixed together, but the pro- 
portion is remarkably constant. In addition to these two 
main constituents, the atmosphere always contains a cer- 
tain quantity of carbon dioxide, some water vapour, and 
generally traces of nitric acid, ammonia, particles of dust, 
micro-organisms and other impurities. 

Oxygen. Oxygen, which forms about one fifth part of 
the total bulk of dry air, is a chemical element, i.e., it 
cannot be split up or resolved into anything simpler. It 
is a very active substance. It combines readily with other 
elements forming compounds called oxides. The process 
is called oxidation. When it takes place heat is given 
out, and, if it is very rapid, flame is produced. Ordinary 
combustion is simply a process of rapid oxidation. When 
oxidation takes place more slowly the temperature does 
not .rise so high, but the material results are the same 
the elements unite with oxygen to form oxides identical 1 
with those produced by burning. The respiration of 
animals and decay of organic matter in the soil are 
examples of slow oxidation. The bodies of animals 
and all kinds of organic matter consist largely of the ele- 
ments carbon and hydrogen. When they are oxidised, 
oxide of carbon and oxide of hydrogen (water) are formed, 
whether the process takes place rapidly by burning or, 
more slowly, by breathing or decay. Plants also are sub- 
ject to a process analogous to the breathing of animals. 
They take up oxygen from the air and give off oxide of 
carbon. This phenomenon is not noticeable in daylight 
as it is then obscured by a reverse action. The reverse 
action is one of de-oxidation ; the plants take up oxide 
of carbon and give off oxygen. 

1 In some cases other oxides are formed. 



12 SOILS AND MANUEES 

Nitrogen. Nitrogen, which constitutes the remaining 
four-fifths of the bulk of the air, is, like oxygen, an element 
or simple body, but it exhibits very different properties. 
Whereas oxygen is one of the most active elements known, 
nitrogen is one of the most inert. The former unites 
readily with other elements ; the latter does not. Many 
substances burn readily in oxygen ; none of them will burn 
in nitrogen at all. Animals and plants absorb and com- 
bine with the oxygen of the air, but, although they take 
in four parts of nitrogen along with each part of oxygen, 
and although they require large quantities of it, they are 
nevertheless quite unable to utilise the free nitrogen of 
the air in this way (p. 8). 

But, though compounds of nitrogen cannot be formed, 
like those of oxygen, by burning substances in air or in 
pure nitrogen gas, such compounds are known and can be 
prepared in various ways. Electric discharges in the air 
thunder cause minute quantities of nitrogen and oxygen 
to unite together forming oxide of nitrogen which, with 
water, produces the traces of nitric acid found in the air. 
The formation of nitric acid by the passage of electric 
sparks through air can be experimentally demonstrated by 
means of a small battery and induction coil. With greater 
electric power more considerable quantities can be formed, 
and a process has recently been devised for the manu- 
facture of nitric acid, on a commercial scale, in this way. 

Ammonia, the most important compound of nitrogen 
and hydrogen, can also be produced by a current of 
electricity passing through a mixture of the two gases. 
Only minute traces of hydrogen are, however, found in 
the air, 1 and the compound is probably not formed in this 
way in Nature. 

A small quantity of ammonia is, however, generally 

1 Dewar, Nature, December 20th, 1900. 



INTBODUCTOBY 13 

present in the air. It is derived from the incomplete com- 
bustion of coal, decomposition of organic matter, etc. Some 
of it is oxidised, forming nitric acid ; some, perhaps a con- 
siderable quantity, is directly absorbed by the soil. The 
remainder of the ammonia, together with all the nitric 
acid and any other compounds of nitrogen present in the 
air, are removed by solution in rain water, by which they 
are carried down, and so ultimately reach the soil. The 
amount of nitrogen gained by the soil in this way, how- 
ever, is probably not very great (p. 16). 

Nitrogen does not unite directly with carbon under ordi- 
nary conditions, but, in the presence of a third substance, 
they may be brought into combination. It has long beer 
known that when carbon is heated with potassium car- 
bonate, and nitrogen gas is passed into the mixture, potas- 
sium cyanide is formed. This substance contains a 
compound of carbon and nitrogen. More recently it has 
been discovered that similar compounds are produced when 
lime and carbon are heated together in an electric furnace 
in the presence of nitrogen, and that these compounds may 
be used as a source of nitrogen for nourishment of plants. 

It is evident that electricity greatly facilitates the forma- 
tion of compounds of nitrogen, and it may be asked whether 
the element could not be rendered directly available to 
plants by this means. Some observers have held that this 
question is to be answered in the affirmative. It is known 
that electric currents have a stimulating effect on the 
growth of plants, but it is not clear that this effect is due 
to the direct assimilation of nitrogen. 

Under natural conditions nitrogen enters into combina- 
tion with other elements through the agency of certain 
vegetable organisms. It is true, as has been previously 
mentioned, that the higher plants, such as agricultural 
crops, cannot absorb and utilise the free nitrogen of the 
air. But it appears that certain simple plants called algae 



14 SOILS AND MANUEES 

and some kinds of bacteria, which are normally present 
in the soil, possess the power of fixing the free nitrogen. 
The compounds so formed undergo various changes and 
ultimately become available to the higher plants. Certain 
other kinds of bacteria, which are also commonly present, 
are able to enter into a symbiotic association with plants 
belonging to the order leguminosse, the pea and bean 
family, and in that condition produce compounds of 
nitrogen which nourish the plant. This, of course, is a 
striking exception to the general rule that plants do not 
utilise the free nitrogen of the air. It is of great import- 
ance, and will be more fully considered later on. The 
facts have been ascertained by numerous careful experi- 
ments, and confirmed by observations on a large practical 
scale. At Bothamsted, it was found that nitrogen accumu- 
lated in pasture soils at the rate of about 50 Ibs. per acre 
annually. In arable soils, under a four-course rotation, 
on an average of forty years, the quantity of nitrogen 
annually removed in the crops exceeded that supplied in 
the manures by some 32 Ibs. per acre. The nitrogenous 
compounds in the organic matter thus accumulated in 
the soil (gradually undergo decomposition, the nitrogen 
is converted into nitric acid and so becomes available for 
the growth of plants. 

The air, therefore, is the ultimate source of all the 
nitrogen, though plants cannot as a rule avail themselves 
of it directly. 

Water Vapour. The proportion of water vapour in the 
air depends upon the temperature and pressure at the time 
and place, and is therefore extremely variable. It is 
increased by evaporation from the soil and free water sur- 
faces. It is diminished by condensation, whereby dew and 
clouds are formed. Clouds do not consist of water vapour, 
properly so-called, but of fine, suspended particles of liquid 
water. It is necessary, therefore, when dealing with the 



INTRODUCTORY 15 

constituents of the atmosphere to eliminate the water, and 
except when otherwise stated, remarks on this subject 
generally refer to dry air. 

Carton Dioxide. This substance, which is present to 
the extent of four parts in ten thousand of dry air, i.e., 
0'04 per cent., is not, like oxygen and nitrogen, an element. 
It is a compound body, and, as the name implies, it con- 
tains the elements carbon and oxygen chemically united. 
It is produced by the burning of fires, the respiration of 
animals and other processes of oxidation of carbon and 
carbon compounds. It is frequently, though not with strict 
accuracy, called carbonic acid because it combines with 
lime and other bodies of that class forming carbonate of 
lime, etc. The carbon and oxygen are very firmly united 
and are not easily separated. All green plants, however, 
can decompose the compound. When exposed to bright 
sunlight they take up the carbon dioxide from the air and 
disunite the elements, giving off oxygen and retaining 
the carbon to build up their tissues. 

Impurities. The impurities in air may be estimated both 
qualitatively and quantitatively by examination of the rain 
water in which they are brought down either mechanically 
or in solution. Besides the nitric acid and ammonia pre- 
viously mentioned, they consist mainly of the chlorides 
and sulphates of sodium, magnesium and calcium all of 
which are more or less soluble some oxide of iron, alu- 
mina and silica, which are insoluble. In addition to these 
inorganic particles there are also numerous micro-organ- 
isms, such as those which cause the souring of milk, 
various kinds of decay and putrefaction, and certain 
diseases such as tuberculosis. 

The following table shows the quantity of nitrogen in 
the form of ammonia and of nitric acid in the rain water 
at various places. The results are given both as parts per 
million and as Ibs. per acre. 



SOILS AND MANUBES 



QUANTITY OF NITROGEN AS AMMONIA AND NITRIC ACID IN THE 
KAIN WATER AT VARIOUS PLACES, PER ANNUM. 1 









Nitrogen per 
million as 


Total 


Place. 


Year. 


Rainfall. 




Nitrogen 












pr acre. 








Ammonia. 


Nitric Acid. 








Inches 






Lbs. 


Kuschen 


1864-5 


11-85 


0-54 


0-16 


1-86 


55 


1865-6 


17-70 


0-44 


0-16 


2-50 


Insterburg . . 


1864-5 


27-55 


0-55 


0-30 


5-49 


55 


1865-6 


23-79 


0-76 


0-49 


6-81 


Dahme 


1865 


17-09 


1-42 


0-30 


6-66 


Eegenwalde 


1864-5 


23-48 


2-03 * 


0-80 


15-09 


55 


1865-6 


19-31 


1-88 


0-48 


10-38 


55 


1866-7 


25-37 


2-28 


0-56 


16-44 


Ida-Marienhiitte, 












mean of 6 years 


1865-70 


22-65 








9-92 


Proskau 


1864-5 


17-81 


3-21 


1-73 


20-91 


Florence 


1870 


36-55 


1-17 


0-44 


13-36 


55 


1871 


42-48 


0-81 


0-22 


9-89 


55 


1872 


50-82 


0-82 


0-26 


12-51 


Vallombrosa 


1872 


79-83 


0-42 


0-15 


10-38 


Montsouris, Paris 


1877-8 


23-62 


1-91 


0-24 


11-54 


5) > 1 


1878-9 


25-79 


1-20 


0-70 


11-16 


" 


1879-80 


15-70 


1-36 


1-60 


10-52 


Mean of 22 years 





27-63 








10-23 



The following 2 is also given as the mean of thirty-nine 



analyses of rain water: 

Total solid matter 

Organic carbon 

Chlorine . 

Nitrogen as organic matter . 

,, ammonia . 

,, nitric acid 

Total combined nitrogen 



Parts per million. 

29-5 
0-7 
2-2 
0-15 
0-24 
0-03 
0-42 



0-27 



1 Fream's 

2 Button's 



Soils and their Properties." 
Volumetric Analysis." 



INTKODUCTOKY 17 

Calculated to Ibs. per acre on a basis of 31 inches 
average annual rainfall, these results correspond to the 
following quantities: 

Lbs. per acre. 

. 1-05 

1-68 1 2-94 



Nitrogen as organic matter 
ammonia . 



,, nitric acid 

Chlorine as common salt 



0-21 
25-37 



From analyses of the rain water collected at Eothamsted, 
in Hertfordshire, the following results 1 were obtained as 
the mean of observations extending over several years : 



Nitrogen as ammonia . 

,, nitrates and nitrites 

Organic nitrogen 
Chlorine as common salt 
Sulphuric acid . 



Lbs. per acre. 
. 2-4} 
. 1-0 4-4 
. 1-0 ) 
. 24-0 
17-0 



The proportions of chlorides and sulphates in rain water 
are extremely variable. They are always much greater 
at the coast and in the neighbourhood of large towns than 
in inland country places. The following quantities have 
been observed : 






Lbs. per acre. 


Chlorides as Common 
Salt. 


Sulphates as Sulphuric 
Acid. 


Inland country 
Coast country 
Manchester .... 
Glasgow 


39-2 
149-8 
66-5 
103-6 


14-7 

53-9 
313-6 
491-4 



Warington, " Chemistry of the Farm.' 



S.M. 



CHAPTEE II 

THE ORIGIN OF SOILS 

THE principal object in describing the origin and 
formation of soils is to account for the presence of the 
various constituents physical and chemical, organic and 
inorganic and to show how the supplies of plant food are 
maintained. 

The soil is obviously derived from the material under- 
neath. This frequently consists of solid rock, and the 
connection between it and the soil can generally be traced 
by examination of the intermediate layers. The rock is 
usually cracked and fissured towards its upper limit, and 
above this it is crumbled and broken into fragments ; these 
are of various sizes, but gradually become smaller and 
finally merge into soil. The topmost layer (some 6 or 
8 inches deep) is usually of a darker colour owing to 
the accumulation of organic matter. In cultivated soils 
the 1-ine of demarcation is often very sharply defined, but 
in other respects the surface layer and the subsoil are 
usually much alike. 

In some cases the soil is formed, by comparatively slight 
modification, from alluvium or drift materials which have 
been transported from a distance the former by the action 
of running water and the latter by glacial ice. These 
deposits are, of course, derived from rocks, and may be 
regarded as intermediate between them and the soil which 
is the final product, but they are sometimes very thick and 
widely spread, and are themselves regarded by geologists 
as rocks. 



THE OEIGIN OF SOILS 19 

Before proceeding to discuss the properties of soils, 
therefore, it is necessary to briefly consider the nature 
of the rocks, the minerals of which the latter chiefly con- 
sist, and the changes by which alluvium, drift material 
and sedimentary soils are formed from them. 

MINERALS. 

Minerals are found in a pure state in small crystals or 
fragments, sometimes in larger lumps or boulders, and 
occasionally in enormous masses which are then called 
rocks. Bocks, however, are usually composed of a mixture 
of minerals, and it is in that form that minerals are most 
widely distributed. Quartz and silicates of potash, soda, 
magnesia, lime and alumina are amongst the commonest 
rock-forming minerals, but oxides, sulphides, fluorides, 
carbonates, sulphates, phosphates and other compounds are 
also present in larger or smaller quantities. The following 
may be taken as typical examples of the different groups. 

Magnetite. Magnetic oxide of iron, Fe 3 04, is a heavy 
black substance, easily converted by oxidation into the 
common red oxide, Fe20 3 . It occurs in small grains in 
volcanic rocks and also massive. 

Haematite. Ferric oxide, Fe20 3 , is very widely dis- 
tributed. It occurs massive in two varieties, known as 
the red and brown haematite respectively, and is the com- 
mon iron ore. It also enters into the composition of rocks, 
to which it imparts its colour. The red colour of soils is 
generally attributable to the presence of ferric oxide. This 
does not necessarily consist of haematite, but may be 
derived from magnetite and other iron-bearing minerals 
originally present in the rocks. 

Limonite is a hydrated ferric oxide ; it occurs in many 
sedimentary rocks and often serves to cement the particles 
together. It is formed by oxidation of ferrous salts result- 

c 2 



20 SOILS AND MANUEES 

ing from the action of carbonic and organic acids on ferru- 
ginous minerals. The rusty deposit often seen in the 
drainage water from moorland consists of limonite. 

Pyrites. Sulphide of iron, FeS 2 , is easily recognised 
by its peculiar golden lustre. It is often present in slates and 
shales and sometimes in clays, to which it imparts a greenish 
tint. In large quantities it is poisonous to vegetation. 

Fluorite. Calcium fluoride, CaF 2 . 

Calcite. Calcium carbonate, CaC0 3 . Arragonite has the 
same chemical composition, but differs in crystalline form. 
Limestone and chalk also consist of calcium carbonate, but 
are of organic origin and are not regarded as minerals. 

Gypsum. Calcium sulphate, CaS04 + 2H 2 0, contains 
water of crystallisation and is perceptibly soluble in water. 

Apatite. Several varieties are known ; the commonest, 
if not decomposed, may be represented by the formula 
3Ca 3 P 2 08 + CaF 2 . This is called fluor apatite to distin- 
guish it from those in which the fluorine is wholly or partly 
replaced by chlorine, and which are known as chlor- 
apatites. When pure the crystals exhibit a blue or green 
colour, but some are grey, white, or colourless and trans- 
parent. Apatite enters into the composition of rocks ; it is 
also found massive, and is fairly widely distributed. 

Quartz. Quartz, a crystalline form of silica, Si0 2 , is one 
of the commonest of all minerals. The white pebbles found 
on the seashore and the large white boulders often seen 
on mountains consist of quartz. This variety is, from 
its appearance, often called "milk quartz." Tinted 
varieties, e.g., "rose quartz," "amethyst quartz," 
" smoky quartz," etc., are also common. The colours are 
due to the presence of traces of metallic oxides. " White 
sand " is simply powdered quartz, i.e., it consists of frag- 
ments of the crystals. Quartz also enters largely into 
the composition of many common rocks, both igneous and 
sedimentary. 



THE OEIGIN OF SOILS 21 

Olivine. Essentially a silicate of magnesia, but part of 
this base is often replaced by iron, and it is generally 
represented by the formula MgFeSiC^. The name is de- 
rived from its olive green colour. It occurs in the volcanic 
rocks, and also, to some extent, massive. 

Mica. A familiar mineral, easily recognised by its 
translucent appearance, pearly lustre, softness, and tend- 
ency to split into flakes. The commoner of the two 
varieties known as white, or potash, mica is a hydrated 
(acid) silicate of potash and alumina ; the formula 
KH 2 Al 3 Si30i2 has been ascribed to it. It is a common 
constituent of granitic rocks, and is found also in some 
of the sedimentary rocks. The large proportion of potash 
(about 9 per cent.) it contains gives it a certain agricultural 
importance. Micaceous sand has been used as a dressing 
for land deficient in potash. The other variety, called 
black, or magnesia mica has a somewhat different com- 
position, the alumina being in part replaced by magnesia. 
It contains about the same proportion of potash but is 
harder than the white variety. 

Leucite. A characteristic ingredient of many of the 
more recent lavas. The formula K 2 Al 2 Si40i2 is ascribed to it. 

Aucjite. Essentially a silicate of lime and magnesia; is 
a very complex mineral ; it enters largely into the composi- 
tion of volcanic rocks and imparts a calcareous (limey) 
character to them. Diallage has much the same composi- 
tion and properties. 

Hornblende. A. silicate of the same type as augite ; it 
contains lime, magnesia, iron and alumina as bases, and is 
found chiefly in volcanic rocks. It is usually very dark 
in colour and exhibits a metallic lustre. 

Felspars. The commonest of all minerals ; are essen- 
tially silicates of alumina and potash, soda or lime. Ortho- 
clase, the potash felspar, is represented by the formula 
KAlSi 3 8 . It is white or greyish in colour, enters largely 



22 SOILS AND MANURES 

into the composition of granite and similar rocks, and is 
very difficult to decompose. Albite, the soda felspar, 
resembles it closely, but contains soda in place of potash. 
Anorthite, the lime felspar, differs more widely ; it is 
represented by the formula CaAl 2 Si20a, occurs chiefly 
in the volcanic rocks and decomposes more readily. 

Nepheline. A silicate of soda and alumina, with some 
potash ; replaces felspars in some lavas. 

Kaolin. A hydrated silicate of alumina, Si20 5 Al 2 (OH) 4 , 
is produced along with potassium carbonate and 
hydrated silica by the weathering of orthoclase. Pure and 
dry, it forms an impalpable white powder which, when 
mixed with water, is converted into a plastic mass. The 
mixed water is easily evaporated ; at higher temperatures 
the combined water is also driven off and does not reunite 
with the residue. Kaolin is used in the manufacture of the 
finer kinds of pottery and is called "china clay." It is not, 
however, to be confused with ordinary agricultural clay. 
The latter is often of very different nature, but usually 
contains some kaolin, and owes its plasticity partly to the 
presence of that substance. 

Zeolites. A large group of hydrated silicates of alumina 
and alkalis or alkaline earths ; are so called from the froth- 
ing or boiling appearance due to evolution of water when 
they are strongly heated. They are usually soft and exhibit 
a pearly lustre ; are probably derived from felspars or 
nepheline, and apparently have been deposited from solu- 
tion. They are found in the veins and cavities but do not 
enter into the composition of rocks. The phenomena of 
absorption have been attributed to the presence of these 
minerals in soils. The formula CaAl 2 Si 6 Oi 6 + 6H a O 
has been ascribed to sfcilbite, and NflaAlaSieOie + 4H 2 
to natrolite. 

The potash-bearing silicates, such as mica, orthoclase, 
etc., occur most plentifully in the granites and rocks of 



THE OEIGIN OF SOILS 



23 



that class; those rich in lime, e.g., augite, hornblende, 
etc., are found chiefly in the volcanic rocks. 

The following tables show the average chemical composi- 
tion of some of the commoner rock-forming minerals and 
the proportion in which, it is estimated, they enter into 
the composition of the crust of the earth. 

AVERAGE CHEMICAL COMPOSITION OF MINERALS. 





Soda. 


Potash. 


Mag- 
nesia. 


Lime. 


Oxide of 
Iron.i 


Alumina. 


Silica. 




Per cent. 


Per cent. 


Per cent. 


Per cent. 


Per cent. 


Per cent. 


Per cent. 


Talc 








33-1 











62-0 


White mica 





9-2 








4-5 


36-8 


46-3 


Black mica' 2 





9-7 


19-0 





14-0 


18-5 


36-7 


Augite 








12-9 


20-9 





6-7 


47-6 


Hornblende 








19-1 


14-0 


3-5 


11-5 


46-3 


Orthoclase 





17-0 











18-4 


64-0 


Albite 


11-8 








. 





19-6 


68-6 


Anorthite 











20-0 





36-9 


43-1 



AVERAGE MINERALOGICAL COMPOSITION OF THE CRUST OF THE EARTH, 



Felspars . . . ... 

Quartz ..... 

Micas ..... 

Talc 

Carbonates of lime and magnesia 
Amphiboles (Hornblende) 3 ^ 
Pyroxenes (Augite) 
Olivine } 

Clay 

All other minerals 




100 



1 Calculated as Fe 2 O 3 . 

2 Very variable. 

8 Arnphibole and Pyroxene are the names of the groups to which 
hornblende and augite respectively belong. Each group includes 
many minerals besides the types mentioned. 



SOILS xlND MANURES 



AVERAGE CHEMICAL COMPOSITION OF THE CRUST OF THE EARTH. 



AS ELEMENTS. 



Oxygen 
Silicon 
Aluminium 

Iron . 

Calcium . 

Magnesium 

Potassium . 

Sodium 

Titanium . 

Carbon 

Hydrogen . 

Phosphorus 

All other elements 



OXYGEN COMBINED. 



Per cent. 




Per cent. 


. 47-2 


Silica .... 


. 57-9 


. 27-2 


Alumina 


. 14-8 


. 7-8 


Oxides of iron 


. 7-8 


. 5-5 


Lime .... 


. 5-3 


. 3'8 


Magnesia . 


. 4-5 


. 2-7 


Potash 


. 3-0 


. 2-4 


Soda . . 


. 3-2 


. 2-4 


Oxide of Titanium 


5 


3 


Carbon dioxide . 


7 


2 


Water 


. 1-8 


2 


Phosphorus pentoxide 


2 


1 


Etc 


3 


2 







100-0 



100-0 



EOCKS. 

Bocks may be primarily classified as igneous, aqueous 
and organic, according to their origin. 

Igneous Rocks. Igneous rocks consist of minerals which 
have been apparently reduced to a state of flux by intense 
heat and solidified in the crystalline form when cooled down. 
In rocks which have cooled slowly, e.g., granite, the 
crystals are large and can be seen with the naked eye. In 
those which have cooled rapidly the crystals are smaller 
and often can only be distinguished by microscopic ex- 
amination. In obsidian the minerals are not crystalline, 
and the rock presents the appearance of a vitreous glassy 
mass. 

Aqueous Rocks. Aqueous rocks were probably formed, 
in the first instance, out of the debris resulting from the 
disintegration of igneous rocks of which, according to the 
nebular hypothesis, the original crust of the earth entirely 



THE OEIGIN OF SOILS 25 

consisted, and subsequently out of the remains of previous 
rocks of aqueous as well as of igneous origin. It is toler- 
ably certain that the original crust of the earth has entirely 
disappeared, and that the igneous rocks of the present day 
are of more recent origin. The processes are still in opera- 
tion. Both igneous and aqueous rocks are constantly being 
formed ; they suffer erosion and denudation, and new 
aqueous rocks are again formed from the remains of both. 

Erosion of the earth's crust is effected by glaciation and 
by weathering. 

Glaciation. Glaciers, in their progress, detach frag- 
ments of rock and carry them down, crushing and grinding 
them by the weight and movement of the ice. The stones 
and boulders brought down with the finely pulverised 
material are usually rounded and scratched. In some cases 
the detritus has been carried considerable distances and 
has accumulated in places to a depth of many feet. It is 
called glacial drift or boulder clay. 

Weathering. The weathering of rocks is caused by 
changes of temperature, frost, water, carbonic acid, and 
oxygen acting more or less continuously for long periods 
of time. 

Changes of temperature cause expansion and contraction 
of rocks as of other substances, the particles become gradu- 
ally loosened, and, finally, the forces which hold them 
together are overcome. It is not any particular tempera- 
ture neither heat nor cold but the frequent change from 
one to the other that ultimately causes the rock to crumble. 
The greatest effects are produced in hot dry climates, in 
which the widest difference between the temperatures of 
day and night occur. The formation of sandy deserts in 
tropical regions is probably due mainly to this cause. In 
those climates the other weathering agents are compara- 
tively inoperative. 

Water assumes its maximum density at the temperature 



26 SOILS AND MANtJBES 

of 4 C., i.e., just before it freezes, and the force of expan- 
sion produced when it solidifies on further cooling is prac- 
tically irresistible. Water lodges in the cracks and inter- 
stices of rocks and is absorbed into the mass of those which 
are porous, and produces tremendous disintegrating effects 
when it freezes. Gigantic masses are split and rent, 
smaller fragments crumble to powder, and tiny particles 
are reduced to dust. It is not the duration or severity of 
the frost but the alternation of periods of frost and thaw 
that produces the greatest effects. Frost is probably the most 
powerful of all the weathering agents, and, in cold, moist 
climates, is the principal cause of the crumbling of rocks. 

Water acts both mechanically and chemically. It is 
proverbial that constant dropping wears away stone. The 
friction of the water and of stones, one against the 
other in streams, rubs down all angularities and leaves 
the stones smooth and rounded. Few minerals are abso- 
lutely insoluble in water, and though the process of solu- 
tion may be slow it is constantly going on, and considerable 
quantities of material are removed in this way. 

Carbonic acid greatly increases the dissolving power of 
water. Many substances, e.g., carbonate of lime, which 
are practically insoluble in pure water, are readily dis- 
solved in water containing carbonic acid, and even hard 
intractable substances like felspar are slowly attacked. 

Oxygen combines with all oxidisable minerals, e.g., mag- 
netite, exposed to its action. The products are generally 
softer and more friable or more soluble than the original 
substance. Oxidation of sulphur-bearing minerals pro- 
duces sulphates, and in some cases sulphuric acid, a 
powerful solvent. 

The ultimate effect of all these forces, acting jointly or 
separately for long periods of time, is to produce physical 
and chemical disintegration of the hardest and most durable 
rocks. They operate on large masses and small fragments, 



THE ORIGIN OF SOILS 



27 



always pulverising them and promoting chemical decom- 
position of the minerals of which they are composed. 

Denudation. The debris resulting from weathering may 
remain in situ, but is often carried away by running water 
or, in dry climates, by wind. 

Wind-blown sands sometimes accumulate in enormous 
quantities, and have been known to cover up tracts of 
fertile country to a depth of several feet. 

Running water is by far the most important agent .of 




FIG. 2. Section showing the Conversion of Rock 
into Soil. 

denudation. The material washed down by rain is carried 
by streamlets to the rivers and so to the sea if not deposited 
on the way. The power of water to carry solid particles 
in suspension increases with the speed of the current. 
When the speed diminishes the larger particles are 
deposited, and, if it becomes slow enough, even the finest 
will ultimately settle out. The course of rivers is often 
diverted by the accumulation of detritus at various points, 
and in time new land is formed, as may be seen in many 
valleys. In a similar manner deltas are formed at the 
mouths of rivers. The current is retarded where it meets 



28 SOILS AND MANUEES 

the sea or lake into which it flows. The channel becomes 
filled up with sediment, and the stream spreads itself out 
in numerous smaller rivulets. These, in turn, become 
blocked by the silt and the process is repeated indefinitely. 
The delta of the Ganges has an area of some 40,000 square 
miles, and at Calcutta the deposit is at least 500 feet thick. 
Great deltas have been formed at the mouths of the Nile, 
Ehine and other rivers. The Holderness in Yorkshire 
has been formed in this way, and the Wash is being gradu- 
ally filled up. 

A deposit of river silt is called alluvium, whether 
formed at the mouth of a river or along its course. 
Alluvial deposits give rise directly to important classes 
of soils which, owing to their mixed character, are usually 
very fertile. In past ages it has frequently happened, 
as a result of various geological changes, that alluvial 
deposits have become cemented together, hardened by pres- 
sure and so converted into what are now called sedimentary 
rocks. Owing to the more or less complete separation of 
the larger from the smaller particles, which takes place 
in the course of their formation, the sedimentary rocks 
can be conveniently classified according to their texture. 
Those which consist of the finest material (clay) are 
described as argillaceous, and those of coarser (sandy) 
material as arenaceous rocks. 

The soluble matter taken up by the water consists of 
salts, such as gypsum, common salt, etc. These crystal- 
lise out when the water evaporates and are deposited in 
layers according to solubility. Large deposits of this kind, 
formed by the drying up of lakes, are called solutionary 
rocks, but, as soils are not derived from them, they are 
of no importance in this connection. Both solutionary and 
sedimentary rocks are, it will be seen, of aqueous origin. 

Organic Rocks. Organic rocks are so called because 
they do not consist of minerals at all but of organic 



THE ORIGIN OF SOILS 29 

remains. Limestones and chalk, formed from the accumu- 
lated remains of certain mollusca and other creatures whose 
shells consist of carbonate of lime, are probably the most 
important of those of animal origin. Coal, peat and inter- 
mediate products composed of the remains of vegetable 
organisms may also be regarded as rocks from the geo- 
logical standpoint, and some of them are of considerable 
agricultural importance. 

Metamorphic Rocks. Sedimentary and organic rocks 
which have been subjected to great heat and partly changed 
or metamorphosed, are called metamorphic rocks. 

Stratification. Aqueous and organic rocks, owing to the 
mode of their formation, are found in layers or strata_, 
and are called stratified rocks. The others are unstratified. 
This is usually the primary basis of geological classifica- 
tion and may be presented in tabular form as follows : 

CLASSIFICATION OF BOCKS. 



I. Unstratified Bocks. 

1. Igneous. 

(a] Plutonic granite. 

(b) Volcanic basalt. 

2. Metamorphic marble. 



II. Stratified Bocks. 
1. Aqueous. 



( arenaceous sand- 



(a] Sedimentary < stone. 



\ argillaceous clay. 



(6) Solutionary gypsum. 

2. Organic. 

(a) Animal limestone. 
(&) Vegetable peat. 

The unstratified rocks presently existing are not neces- 
sarily older than the stratified formations, in fact some 
of them are quite recent. They consist of crystalline 
minerals and contain no fossils. Granite and basalt 
respectively may be taken as typical examples of the 
two principal sub-divisions of the igneous group. They 
exhibit, in well marked degree, characteristic differences 




30 



SOILS AND MANUEES 



corresponding to the difference in origin and in mineralogi- 
cal and chemical composition, though, it should be noted, 
there is no sharp line of demarcation between the two 
groups. 

Granite exhibits a coarsely crystalline texture, probably 
because it has been formed beneath the surface and has 
cooled slowly under great pressure. It is composed of 
quartz, orthoclase and white mica. It decomposes slowly, 
but ultimately weathers white. 

Basalt is simply the lava poured forth from volcanoes 
in a molten condition. It has cooled quickly, and the 
crystals of the minerals are, therefore, often so small as 
to give it, superficially, an almost homogeneous appear- 
ance. It is composed of augite, anorthite and black mica, 
and often contains also olivine, enstatite and magnetite, 
which cause it to weather red, but it contains no quartz. 
It is comparatively easily decomposed. 

In consequence of the difference in mineralogical com- 
position granite always contains more potash and silica, 
and is of lower specific gravity than basalt. The soils 
formed from it are stiff, cold and relatively unproductive. 
Basalt is richer in lime and decomposes more readily, 
yielding a lighter and more fertile class of soils. The 
difference in chemical composition will be seen from the 
following analysis : 

AVERAGE CHEMICAL COMPOSITION OF GRANITE AND BASALT. 






Granite. 


Basalt. 




Per cent. 


Per cent. 


Silica (free and combined) . 


72-0 


46-0 


Alumina 


16-0 


16-5 


Oxide of iron .... 


1-5 


14-4 


Lime ...... 


1-0 


12-5 


Magnesia 


4 


6-0 


Potash 


6-9 


1-5 


Soda ...... 


2-0 


3-0 


Phosphoric acid .... 


trace 


2 



THE ORIGIN OF SOILS 31 

The stratified rocks are so called because they are 
deposited in layers or strata. They are derived from 
the remains of previous rocks, though not necessarily 
directly from the unstratified, and frequently are com- 
posed of non-crystalline minerals. They contain fossils, 
and are classified by geologists according to age 
(p. 42). In this country stratified rocks are the most 
abundant, the soils derived from them are widely distri- 
buted and reflect their characteristic properties. Sand 
and clay respectively may be taken as the two chief types 
of the sedimentary deposits. 

Sand. Sands are found plentifully by the seashore, 
sometimes in level stretches covered with stunted verdure, 
sometimes piled up in mounds or dunes fifty or sixty feet 
high. Inland beds of sand, such as the Bagshot sands, 
Thanet sands, etc., are also common and cover large tracts 
on the geological map. It is often said that sand consists 
of silica, and sometimes that silica is pure sand. Unquali- 
fied, the statements are not true, and their repetition has 
led to much confusion. The term sand is commonly applied 
to quantities of small gritty angular fragments of any 
crystalline mineral or mixture of minerals. Quartz 
(crystallised silica) is usually present in larger or smaller 
quantity, and not infrequently comprises the bulk of the 
material. Sea sands often consist entirely of quartz - 
a fact which probably gave rise to the misstatement referred 
to but inland deposits are generally more mixed in 
character. 

The size of the particles varies. Sand is a popular 
rather than a scientific term, and cannot be defined as 
consisting of grains of any particular size. The only limit- 
ing property is that vaguely known as grittiness. This 
property would not be generally recognised in material 
consisting of particles of less than 0'025 millimetres (say 
f an inch) or greater than 2*5 millimetres (say -^ of 



32 SOILS AND MANURES 

an inch) diameter. Such material would be considered as 
merging into clay on the one hand, and into gravel on the 
other. In ordinary sands the grains are not of uniform 
size, and,, though the extremes mentioned above are, per- 
haps, not often reached, they may vary from about 0*5 
to 0'05 m.m., or say, roughly, from ^ to ^J a ^ an ^ nc ^ 
in diameter. Sand is often described as fine, medium or 
coarse, according to the size of the grains ; the terms are 
merely relative and are self-explanatory. Nearly all the 
more important physical properties of sand can be traced 
to the size and hardness of the particles. 

In the sense of mass, sand is a heavy substance, i.e., it 
weighs more than an equal volume of more finely pulverised 
material of similar density. Thus a cubic foot of sand 
weighs about 90'3 Ibs., whereas a cubic foot of clay weighs 
only 68 Ibs. 

In the agricultural sense sand is " light." In this case the 
expression refers to the " tenacity " or lack of it which sand 
exhibits, and which makes sand light or easy to cultivate. 

Corresponding to the size of the grains, the spaces 
between them are also large and cannot therefore retain 
much water. When the water drains away its place is 
taken by air, and consequently sands are usually in a high 
state of oxidation. But if the individual spaces are large 
there are not so many of them, and the total amount of 
unoccupied space is not so great as in substances which 
consist of smaller particles. 

Sands are not usually associated with large amounts 
of plant food. Quartz itself contains none, and whatever 
plant foods are present are derived from other minerals. 
Some deposits of sand, however, contain constituents of 
plant food in quantities so large that they can be usefully 
employed as dressings for cultivated soils. Conspicuous 
amongst such are the previously mentioned micaceous 
sand, which has been used as a source of potash; shell 



THE ORIGIN OF SOILS 



33 



sand, calcareous sand and marly sand, all of which are 
valuable for the lime they contain. 

Sandstones are formed of particles of sand cemented 
together. The matrix, or cementing material, is described 
as calcareous, ferruginous, siliceous or argillaceous, ac- 
cording as it consists of lime, oxide of iron, silica or clay. 
Phenomena of this kind sometimes occur in cultivated soils 
and are called "pan formations." They may be due to 
other causes, but are generally an indication of a tendency 
of soils to revert to rock. 

Clay. Clay beds, such as the Oxford clay, London clay, 
etc., are widely distributed, and cover large tracts on the 
geological map. It is often said that clay consists of kaolin, 
but the term is applied indiscriminately to quantities of 
any mineral or mixture of minerals reduced to an impalp- 
able powder. In this condition the particles, when wet> 
cohere together and give the mass that vague quality of 
plasticity which is the characteristic property of clay. The 
difference between clay and sand is, in the main, a ques- 
tion of size of the particles. The most finely pulverised 
minerals are, however, more readily decomposed than those 
of larger size, and, as might be expected, have a somewhat 
different chemical composition. 

Analysis l of the particles of different sizes separated 
from the same sample of soil gave the following results : 



Size of Particles. 


Silica. 


Alumina. 


Oxide of Iron. 


m.m. 








2 '04 


94-6 


3-4 


1-1 


04 -01 


92-0 


6-2 


1-2 


01 -004 


88-3 


8-5 


1-8 


004 -002 


61-7 


23-4 


7-0 


002 


45-9 


30-9 


12-2 



S.M. 



1 Hall, " J C. S. Trans., 1904.' 



34 SOILS AND MANURES 

The analysis of the smallest particles given above is the 
mean of several samples of soil, all of which gave very 
similar results. This material prohahly consists of about 
75 per cent, of kaolin mixed with about 25 per cent, of 
other minerals, chiefly quartz and oxide of iron. The 
particles larger than O004 m.m. diameter evidently con- 
sist mainly of undecomposed minerals, in this case quartz. 
It appears, therefore, that in chemical composition sand 
and clay may differ widely or they may be much alike. 
The characteristic of the one is grittiness, and of the other 
impalpability, or, when wet, plasticity. These properties 
have reference to the sense of touch, and it is impossible 
to fix upon any precise limit of demarcation. As the 
particles become finer the material loses the character 
of sand and takes on that of clay. Whatever the minimum 
limit of size of the particles may be for sand, that will 
be the maximum limit for clay. It has been suggested 
that this limit will generally be recognised somewhere 
about 0*025 m.m. diameter. The finest particles of clay 
are indistinguishable under ordinary powers of the micro- 
scope, and it is not known what the ultimate limit of 
division may be. It has recently been discovered, however, 
that some, hitherto indistinguishable, can be rendered 
visible by the use of stains or dyes, and, by using high 
power objectives, with oil immersion, particles of O'OOOlm.m. 
diameter have been successfully measured. The average 
diameter of the particles in the finest group that can be 
separated by processes of elutriation is about 0'002 m.m. 

The coherence, plasticity and other characteristic pro- 
perties of clay depend mainly upon the proportion of the 
more minute particles, and, to some extent, upon the pre- 
sence of gelatinous hydrates of iron, alumina and silica 
and colloidal organic compounds. 

In the sense of mass, clay is a light substance ; its true 
density is only 2*5, whereas that of quartz is 2*62, and 



THE ORIGIN OF SOILS 35 

owing to its state of fine division, it weighs much less than 
an equal volume of sand (vide ante). In the agricultural 
sense it is "heavy " or difficult to cultivate owing to the 
coherence of the particles. 

Corresponding to the minute size of the particles, clay 
exhibits a great internal surface and a finely porous char- 
acter. It can, therefore, absorb a larger quantity of water 
and retain it more strongly than material of coarser texture. 
The presence of the water hinders the entrance of air and 
so retards oxidation. 

Clays are derived chiefly from granites and similar rocks. 
They consist of the most finely pulverised sediment, and 
some of the minerals are usually in an advanced state of 
decomposition. Clay, therefore, usually has associated 
with it a certain amount of plant food in a more or less 
readily assimilable condition in addition to that contained 
in the undecomposed minerals. It does not, however, as 
a rule, yield very fertile soils because, in addition to the 
difficulties of cultivation, they are ^apt to be cold, wet and 
poorly oxidised. 

It will be noticed that the unproductiveness of clay soils 
is attributed not to chemical but to physical causes which 
are ultimately dependent upon the size of the particles. 

Analyses of clays, showing the proportion of the 
principal ingredients, are given in the table on p. 36. 

Shales. Shale is simply clay which has become har- 
dened and laminated. It is easily split up into layers along 
the planes of bedding. This characteristic property is 
greatly promoted by the presence of certain minerals such 
as mica, sand, etc. ; calcareous and bituminous matters 
produce a similar effect. The composition of shales is 
very variable. They "merge into clays on the one hand and 
into slates on the other. The slates are of similar character 
but harder. They both yield a very poor class of soil. 

Marl. Marl consists of clay mixed with carbonate of 

D 2 



36 SOILS AND MANUKES 

AVERAGE CHEMICAL COMPOSITION OF CLAYS. 





Pottery Clay. 


Fire Clay. 




Poole. 


Bovey. 


Stourbridge. 


Broseley. 




Per cent. 


Per cent. 


Per cent. 


Per cent. 


Silica 


55-9 


738 


79-1 


68-4 


Alumina . 


36-7 


13-4 


17-1 


22-0 


Lime ... 


5 





trace 


1 


Magnesia 


2 


3-9 


trace 





Potash . 


3-9 





\ fl'Q 


1-0 


Soda 


2-3 





> u y 


4 


Ferrous oxide 





1 a 7 


2-6 


3 


Ferric oxide . 





1 8? 




7-4 



lime, but retaining its plasticity. When partly indurated 
it is called marl shale or marl slate, being easily split into 
layers. The harder varieties are called marl stone. They 
crumble very readily on exposure to weather. In the 
neighbourhood of marl beds, .marl is extensively used as a 
source of lime and produces good effects, especially on the 
lighter soils. The proportion of carbonate of lime varies 
considerably. As it increases the plasticity gradually 
diminishes, and when it becomes predominant the material 
ceases to be regarded as marl and is called argillaceous 
limestone. The following table shows the average 
composition of certain marls : 

AVERAGE CHEMICAL COMPOSITION OF MARLS. 






Chalk Marl, 
Farnham. 


Kimmeridge 
Clay Marl, 
Dorset. 


Jurassic 
Marl, Jura. 


Keuper 
Marl, 
Worcester. 




Per cent. 


Per cent. 


Per cent. 


Per cent. 


Silica and clay . 


26-1 


1 AK.7 


40-2 


53-6 


Alumina and oxide of iron 


3-0 


} DO I 


3-8 


25-4 


Carbonate of lime 


66-7 


34-3 


52-5 


7-7 


Carbonate of magnesia 


2-4 





2-2 


10-4 


Phosphoric acid . 


1-8 





1-3 


2-9 



THE OEIGIN OF SOILS 



Limestone. Limestones are chiefly of organic origin, as 
has been previously pointed out. They consist essentially 
of carbonate of lime, and, in many cases, are nearly pure. 
On the other hand, some of the deposits are associated with 
a large amount of impurity consisting of amorphous silica, 
sand, clay or ferruginous matter. These are called siliceous 
limestones, sandy limestones, argillaceous limestones, and 
so on. The purer varieties are usually white or greyish in 
colour, crystalline and hard enough to be ultilised as build- 
ing stones. Chalk, which is of the same chemical nature, 
is also white, but is dull, soft and friable. The oolitic 
limestone is of different origin and structure. It consists 
of rounded grains of carbonate of lime embedded in a more 
or less calcareous matrix. 

Dolomite, which Consists of the carbonates of calcium 
and magnesium, is regarded as a mineral rather than a 
rock, but it occurs in large rock-like masses and is known 
as magnesian limestone. The following table shows the 
average composition of some of the well-known varieties 
of limestone: 

AVERAGE CHEMICAL COMPOSITION OF LIMESTONES. 






Chalk, 
with 
Flints, 
Kent. 


Shelly 
Lime- 
stone, 
Portland. 


Hydraulic 
Lime- 
stone, 
Kimmer- 
idge. 


Great 
Oolite, 
Bath. 


Magnesian 
Lime- 
stone, 
Bolsover. 


Silurian 
Lime- 
stone. 


Red Chalk, 
Norfolk. 




Per cent. 


Per cent. 


Per cent. 


Per cent. 


Per cent. 


Per cent. 


Per cent. 


Carbonate of 














lime 


98-4 


79-0 


75-7 


94-5 


51-1 


44-6 


82-3 


Carbonate of 
















magnesia . 
Silica . 


1 
1-1 


3-7 
10-4 


15-0 


2-5 


40-2 
3-6 


3-6 
51-4 


11-3 


Oxide of iron 
















and alumina 


4 


2-0 


8-2 


1-2 


1-8 





6-4 


Water, etc. . 





4-2 


1-1 


1-8 


3-3 


4 


__ 



Peat. Peat differs widely from all other rocks and soil- 
forming materials both in nature and properties. It has 



38 SOILS AND MANURES 

accumulated in enormous quantities in certain places. 
Local deposits often extend over an area of several square 
miles, and may be as much as 50 feet thick. It is calcu- 
lated that in Ireland alone over 4,000 square miles are 
covered with peat. Peat bogs are formed by the growth of 
mosses, sphagnum, and other aquatic plants. New genera- 
tions grow and flourish on the surface, and are nourished 




FIG. 3. Section of Peat Bog. 

by the products of partial decomposition of previous 
generations below. The moss accumulates relatively fast 
in some cases. Growths at the rate of about two and a half 
inches annually have been recorded. Examination of a 
section of a bog from which peat has been cut reveals in- 
dications of continuous change from above downwards. 
The topmost layer is usually of a light golden or fawn 
colour ; this merges gradually into dark brown, and 
finally into black at a depth of two or three feet. The 
texture of the material corresponds to the gradations of 
colour ; the upper layers are more fibrous and spongy, the 
lower are harder and more compact. These layers also 



THE ORIGIN OF SOILS 39 

differ in chemical composition. Analysis of a portion 
taken from about a foot below the surface showed that, 
apart from water and ash, it contained 60*1 per cent, of 
carbon, 32'4 of oxygen, 5*1 of hydrogen, and 1/9 of nitro- 
gen. The lower layers contain a larger proportion of carbon 
and less of the gaseous elements. The ash is very variable, 
both in quantity and quality, because it is difficult to 
secure samples .entirely free from adventitious sand and 
other matters not properly constituents of the peat itself. 
The proportion of true ash probably does not exceed 1 per 
cent, of the dry matter, and is of much the same com- 
position as the ash of other plants. 

The physical properties of peat are also peculiar. It 
consists essentially of a mass of decaying vegetable matter, 
and is soft and spongy in character. Its density varies 
from about 1*4 to 1/6, and the weight of a cubic foot of the 
dry matter, uncompressed, is only about 21 Ibs. In the 
dry powdery condition it exhibits but little tenacity; it 
is easily moved and blown about. It is extremely porous 
and has a vast internal surface. Both on this account and 
because it unites with water-forming colloidal hydrates, its 
power of absorbing and retaining water far surpasses that 
of any other soil-forming material. When the pores are 
not blocked with water it can absorb large quantities of 
air, but the oxygen is rapidly withdrawn by combination 
with the substance of the peat itself. 

It has been shown that peat contains a considerable 
proportion of nitrogen. It is the only rock, or soil-forming 
material, which .naturally contains that element as an 
essential constituent. Soils formed from peat are there- 
fore naturally much richer in nitrogen than those of 
mineral origin, but they are generally deficient in all the 
other elements of plant food. 

Summary. It appears, then, that the crust of the ejarth 
is composed of rocks. It undergoes erosion and denuda- 



40 SOILS AND MANUEES 

tion ; the older rocks are destroyed and new ones are 
continually being formed. With the exception of the 
organic rocks, which consist of the remains of animal and 
vegetable organisms, they are composed of minerals. The 
igneous rocks generally consist of crystalline minerals 
fused together, and are very hard. In the sedimentary 
deposits the minerals may be crystalline as in sandstones, 
or amorphous as in slates and shales. They are often 
cemented together by a matrix of some kind, or consoli- 
dated and hardened by pressure. In other cases they are 
comparatively soft. 

Formation of Soils. Kocks of all kinds whatever their 
origin, composition or physical properties undergo 
change at the surface where they are exposed to the action 
of the weather. The harder rocks are disintegrated and 
pulverised, and the minerals are decomposed chemically as 
above described. The softer materials also are loosened, 
oxidised and chemically altered. The pulverent material 
thus produced is, at least physically, capable of acting as a 
medium for the growth of plants and usually contains some 
plant foods derived from the minerals. In addition to the 
effects produced by the mechanical and chemical agents 
frost, water, carbonic acid, etc. vegetable and animal 
organisms play a not inconsiderable part especially in the 
final stages in which the pulverised material is converted 
into soil properly so called. 

The formation of organic matter is probably due, in 
the first instance, to the action of certain bacteria and 
alge which possess the power of assimilating the free 
nitrogen of the air (pp. 187, 142) and which are normally 
present in the soil. Bacteria are found on the surface of 
bare rocks, on exposed mountains, and even the hardest 
stones and rock masses soon become covered over with a 
lichenous growth. The quantity of organic matter so 
formed may be, at first, small, but its presence in the soil 



THE OEIGIN OF SOILS 41 

has not previously been accounted for. The substances 
provided by the decomposition of the plants, after death, 
and the nitrogenous compounds brought down in the rain 
water (p. 17) add to the possibilities of further growth 
and the organic matter is gradually increased. The pene- 
tration of the roots of plants, the solvent action of their 
acid juices and the carbonic acid produced by the decom- 
position of the organic matter, all tend to loosen the par- 
ticles, allow air and water to enter, and so hasten the 
conversion of rock into soil. 

The action pf burrowing animals, such as moles, in 
loosening and stirring the soil is familiar. That of earth- 
worms is perhaps less noticeable but more important. 
These creatures swallow the finer particles of soil for the 
double purpose of making their burrows and of obtaining 
nourishment from the organic matter. Most of this 
material is carried up and deposited at the surface. It 
has been calculated that there are about 25,000 earth 
worms in an acre of average soil, and that, by their com- 
bined action, about ten tons of finely pulverent material 
are brought to the surface every year. According to this 
calculation each animal would have to move about a 
twenty -fifth part of an ounce per day. 

Now all these agencies, mechanical, chemical, and bio- 
logical, are constantly at work producing physical and 
chemical disintegration of minerals and the ..growth and 
decomposition of organic matter. 

Mechanical pulverisation converts the rock into a suit- 
able medium for the growth of plants and accelerates the 
chemical changes in the minerals and organic matter 
whereby their constituents are reduced to a condition in 
which they can be assimilated. These plant foods are sub- 
ject to constant abstraction by crops, and in other ways, 
and they are constantly regenerated by the operation of 
the various forces on the fragments of rock and undecom- 



42 



SOILS AND MANUEES 



posed minerals of which the bulk of the soil is composed. 
The natural fertility of soils, so far as the supply of plant 
food is concerned, depends .upon the rapidity with which 
these changes take place, and it is the object of the various 
operations of cultivation, ploughing, fallowing, etc., to 
promote them as well as to improve the physical properties 
of the soil. 

Classification of Soils. For agricultural purposes soils 
are most conveniently classified according to the nature of 
the predominant constituent. Names such as sandy soil, 
clay soil, calcareous soil, etc., are self-explanatory. Those 
of intermediate texture, between sand and clay, are called 
loamy soils. They are sometimes further qualified as sandy 
loams or clay loams. In a similar manner we may have 
calcareous sands, clays or loams, humous clays, and so on. 

The following tabular ^statement shows the principal 
formations of the stratified rocks with their more impor- 
tant divisions and sub -divisions arranged in geological 
order and the nature of the materials 1 of which the different 
strata are composed : 



Tertiary or Cainozoic. 


Pliocene. 





Weybourn crag and Chilles- 
ford clay, Norwich crag, red 
crag, Suffolk crag, etc. 
shelly and ferruginous sands 
and gravel. 


Miocene. 






Oligocene. 





Hempstead, Bembridge, Os- 
borne and Headon groups 
clays, marls, limestones, 
and sands with shelly 
layers. 



Geikie. 



THE OKIUIN OF SOILS 



43 



Tertiary or 
Cainozoic. 


Eocene. 





Barton clay, Brackle sham beds. 
Bagshot sands. 
London clay. 
Woolwich and Heading beds. 
Thanet sands. 


Secondary or Mesozoic. 


Cretaceous. 


Upper. 


Chalk, with flints (Norwich, 
Brighton, Dover, Flam- 
borough Head). 
Chalk, without flints (Dover). 
Grey chalk (Folkestone), chalk 
marl, red chalk (Hunstanton) . 
Upper greensand. 


Lower. 


Gault (clay). 
Lower green sand. 
Weald (clay). 
Hastings sands. 


Jurassic. 


Upper oolite. 


Purbeck beds. 
Portland limestone, marls and 
sandstones. 
Kimmeridge clay. 


Middle oolite. 


Coralline limestones, cal- 
careous grits and clays. 
Oxford clay stiff blue and 
brown clay, calcareous grits. 


Lower oolite. 


Bath shelly limestones, clays 
and sandstones, sands (corn- 
brash). 
Cheltenham calcareous grits. 


Lias. 


Upper sandy beds, clays and 
shales. 
Middle limestones, sands and 
clays (marlstone). 
Lower blue and brown lime- 
stones and dark shales. 



44 



SOILS AND MANUEES 



Secondary or 
Mesozoic. 


Triassic. 


Upper. 


Bed, grey, and green marls, 
with beds of rock salt and 
gypsum. 
Pted sandstones and marls. 


Lower. 


Mottled, red, or green sand- 
stones, marls and pebble beds. 


Primary or Palaeozoic. 


Permian. 





New red sandstone, clays and 
gypsum. 
Magnesian limestone. 
Marl- slate, sandstones. 


Carboniferous. 


Coal measures. 


Red and grey sandstone, clays 
and limestone. 
White, grey and yellow sand- 
stone, clays and shales. 
Coal-bearing beds. 


Millstone grit. 


Grits, flagstones, sandstones 
and shales, with thin seams 
of coal. 


Mountain 
limestone. 


Massive limestones and shales, 
with sandstones and thin 
coal seams. 


Devonian. 


Upper. 


Pilton and Pickwell Down 
group. 
Upper old red sandstone. 


Middle. 


Ilfracombeand Plymouth lime- 
stones, grits and conglomer- 
ates. 


Lower. 


Linton slates, and Devon and 
Cornwall sandstones. 
Lower old red sandstones. 



THE OEIGIN OF SOILS 



45 



6 
'o 

N 
O 

& 

t4 

O 

j>J 

03 

1 


Silurian. 


Upper. 


Ludlow group (mudstone and 
Aymestry limestone),Kirkby 
Moor and Bannisdale flags 
and slates. 
Wenlock group shales and 
limestones. 
Llandovery group May Hill 
sandstones and Tarannon 
shales. 


Lower. 


Bala and Caradoc group 
sandstones, slates, grits with 
Bala limestone. 
Llandeilo group dark argilla- 
ceous and calcareous flag- 
stones and shales. 
Arenig group dark slates, 
flags and sandstones. 


Cambrian. 


Upper. 


Tremadoc group dark grey 
slates. 
Lingula flags blue and black 
slates, flags and sandstones. 


Middle. 


Solva group. 
Menevian group Sandstones, 
shales, slates and grits. 


Lower. 


Harlech and Llanberis group 
purple and grey flags, sand- 
stones and slates. 



The succession of the strata in the tertiary and secondary 
formations is also illustrated in a diagrammatic fashion in 
the section (Fig. 4). The average thickness of the beds, 
as they occur in this country, is approximately indicated 
by the scale of the drawing and the nature of the materials 
of which they are composed by the shading. 

The characteristic properties of soils correspond more 



'J2^ 1000 Ft (about) 
Pliocene ^ ra g 

n,. Hempstead. Bembridqe 

O/.gocene 



(Bagshot Sands 

Eocene \LondonClay 

\Thanet Sands 
Chalk 



U. Green Sand 

Gault 

L. Green Sand 



Cretaceous 



TrU 



Permian 



Weald 

Hastings Sands 
Purbec if Portland Beds 

Kimmeridge Clay 
Coralline 

Oxford Clay 
Cornbrash 

Bradford Clay 
Stones fie/ d Slate 
Bath Oolite 
Fullers Earth 
Inf. Oolite 
Mid ford Sands 

Mar/stone 
Lower Lias 

Clays & Marls 



/fee/ Sandstone 



Magnesian Limestone 



Lower Permian 



^F 



_L~_T 



Carb on ife rous 



FIG. 4. 



THE OEIGIN OF SOILS 



47 



Carboniferous 




Coal Measures 



Mfllstone Grit 



Yoredale Rocks 



Carboniferous 
Limestone 




l . 1 



I., I 



1,1', I 



It I 



'.'I 



' . ' 



Devonian 



FlG. 4A. 



48 SOILS AND MANUEES 

or less closely with those of the strata from which they 
are derived, and may be judged, to some extent, from 
the nature of the materials of which the latter are com- 
posed. Thus, in general, soils derived from the Crag, 
Eocene, Cretaceous, Permian and Devonian sands are light 
or medium in texture, easily cultivated and usually fertile ; 
in some cases they are very rich. Those derived from the 
carboniferous, silurian and Cambrian formations are also 
light in character, but not usually so fertile as those pre- 
viously mentioned. The London clay, gault, weald and 
Jurassic clays as a rule produce stiff cold land, but when 
well drained and limed these soils are sometimes very pro- 
ductive. Chalk and limestones generally yield soils of very 
inferior quality. 

Soils of mixed character are usually richest in plant 
foods and possess the most suitable physical properties. 
For this reason loams are to be preferred to either sands 
or clays, calcareous sands to either calcareous or sandy 
soils, humous clays to either clay or vegetable soil. 

Soils derived from strata of mixed character are there- 
fore generally more fertile than those formed from more 
homogeneous materials. For example, the soils derived 
from the lower beds of the London clay, in which a quan- 
tity of sand is present, are generally more productive than 
those derived from the upper portion, which consists almost 
entirely of clay. The lower chalk, again, which is of a 
marly character, produces some good fertile land_, whereas 
the upper chalk, which is nearly pure, yields very poor 
^oils. 

For the same reasons, in districts where two formations 
meet, the soils are always more fertile than those derived 
from either formation alone. For example, where the chalk 
mixes with the London clay, which lies above, or with the 
green sand below it, richer and more productive soils 
result than from either the London clay or the green sand 



THE OEIGIN OF SOILS 49 

alone, notwithstanding that the chalk itself is much 
poorer than either of them. 

Alluvial and drift soils also owe their general fertility 
largely to the heterogeneous character of the materials 
from which they are derived. 



S.M. 



CHAPTER III 



THE PHYSICAL PEOPEETIES OF SOILS 
THE PARTICLES 

Size of the Particles. The physical properties of soils, 
like those of their constituents, depend very largely upon 
the size of the particles, and, as a rule, soils are extremely 
heterogeneous in this respect. The coarser particles- 
stones, etc. may be separated and their size determined 
by means of sieves with circular holes of known diameter. 
The size of the particles in the fine earth can be estimated 
by microscopic measurements. The proportion of stones 
and coarse material may vary from 50 per cent, or more 
down to zero ; from 2 to 20 per cent, is common in culti- 
vated soils. The proportions in which the particles of 
various sizes are mixed together in the fine earth of dif- 
ferent soils are still more variable. All that can be said is 
that those of coarser texture contain more of the larger and 
those of finer texture more of the smaller particles. 
Taking particular cases for example, the following table 
shows the proportions in which the particles of various 
sizes were found in the fine earth of some sandy, loamy and 
clay soils respectively : 






Diameter of Particles. 


Sandy 
Soil. 


Sandy 
Loam. 


Clay 
Loam. 


Stiff 
Clay. 




m.m. 


Per cent. 


Per cent. 


Per cent. 


Per cent. 


Gravel 


3-0 1-0 


0-7 


1-8 


3-4 


0-9 


Coarse sand 


1-0 0-2 


45-4 


18-9 


8-9 


1-6 


Fine sand 


0-2 0-05 


39^ 


25-3 


19-6 


14-6 


Coarse silt 


0-05 O'Ol 


d*9 


31-2 


37-2 


36-7 


Fine silt . 


0-01 0-005 


3-2 


9-5 


12-6 


10-8 


Clay 


0-005 


3-4 


13-9 


17-6 


34-9 



THE PHYSICAL PROPERTIES OF SOILS 51 

The names gravel, sand, etc. given to the different 
fractions in the table are quite empirical and, though in 
common use, are not always applied in exactly the same 
sense. It is more important to notice that in the sandy 
soil more than 80 jper cent, of the particles are of greater 
diameter than 0*05 m.m., in the clay soil more than 
80 per cent, of the particles are smaller than that, 
while in the loamy soil the particles are of intermediate 
size. 

Number of Particles. The number of particles in a 
given volume could be easily computed if they were 
regular in shape, size and arrangement. Thus, if the 
particles were cubes and were built up in a compact mass 
without any spaces between them, one cubic decimetre 
would contain a thousand cubic centimetres, a million 
cubic millimetres, and so on. 

If each cube were replaced by a sphere or particle of any 
other shape the number would remain unaltered. A cubic 
decimetre would thus contain a thousand spheres of one 
centimetre diameter, a million of one millimetre diameter, 
and so on. In short, the number of spheres in a given 
volume would be inversely proportional to the cube of the 
diameter. 

If the spheres were as small as the particles of sand say 
T Jo part of an inch in diameter there would be a million 
of them in one cubic inch ; if they were as small as those 
of clay say 5^00 part of an inch in diameter there would 
be 125,000,000,000 in one cubic inch. 

Arrangement of the Particles. - - If spheres were 
arranged in the manner indicated above, by substituting 
them for cubes in a compact mass, the spaces between them 
would be the largest possible. Any re-arrangement would 
therefore bring them closer together, and a larger number 
would be required to fill a given volume. 

Interspace. The amount of space occupied by a mass 

E 2 



52 



SOILS AND MANUEES 



of uniform particles of any given form depends upon the 
arrangement. 

The largest sphere that any cube can contain l occupies 
only 52*361 per cent, of the space, and the remaining 




Fm. 5. 





FIG. 6. 



FIG. 7. 



47'639 per cent, is left unoccupied (Fig. 5). The same 
relation obviously holds good when any number of cubes 
(in a compact mass) are replaced by spheres' (Fig. 6). 

1 The relation between the volume of a cube and the largest sphere 
it can contain is shown by the formulae 
(2r) 3 : 4/3 Trr 8 
cube sphere 

1 : -52361 = 52-361 per cent. 



THE PHYSICAL PROPERTIES OF SOILS 53 

If the spheres were arranged in the form of a pyramid, 
like a pile of cannon balls (Fig. 7), they would lie much 
closer together, the points of contact would be more 
numerous, and a larger number would be required to fill 
a given volume. In such a case the spheres would occupy 
74*1 per cent, of the total volume, and only 25'9 per cent, 
would be left unoccupied. 

This explains why the volume of a loose powder is per- 
ceptibly diminished on shaking and how a soil may be 
consolidated by pressure. The particles undergo re- 
arrangement; they are brought into closer contact and 
the amount of interstitial space is reduced. 

The size of the particles, so long as they are uniform, 
does not affect the relative proportions of occupied and un- 
occupied space. If the particles were smaller so also would 
be the spaces between, but as there would be more of them 
the total amount of occupied and unoccupied space would 
remain unaltered. Thus, if the diameter of the particles 
were divided by a there would be a 3 times the number in 
the same volume, and the space occupied by them would b 
the same. 1 This may, perhaps, be made more plain by 
supposing the mass of particles to be viewed through a 
magnifying glass. The particles and the spaces would be 
magnified in the same proportion, but the relation of one 
to the other would not be affected. 

It is obvious that smaller particles could be introduced 
into the interspaces, and the amount of unoccupied space 
could be thus considerably reduced. Variation in the size 
of the particles therefore tends to reduce the amount of 
unoccupied space. Particles of irregular shape, like those 
of soils, could be arranged more closely than spheres, but 
not so closely as cubes. Any adaptability of shape due to 
softness of the particles would also tend to reduce the 



4/3 TT (r/a)3 = * ~ = 4/3 IT r". 



54 SOILS AND MANURES 

amount of interspace, but that which affects it most is the 
arrangement of the particles. 

The interstitial space in dry soil is filled with air, but the 
air may be wholly or partially displaced by water. The 
magnitude of the individual spaces may be judged from 
the size of the particles. 

The total amount of unoccupied space can be calcu- 
lated from the formula 

*= 100 -(!>*). 

S is the percentage of interstitial space, M the mass of 
soil, V the weight of an equal volume of water, and d the 
true density of the soil. 

Calculating from these data the following results were 
obtained : 

UNOCCUPIED SPACE IN DRY MATERIAL. 

Per cent. 

Quartz sand . 44*7 

Clay ' . 59-5 

Humus 75-0 

Arable soil ......... 53'4 

Old pasture , 64-1 

Internal Surface. The internal surface of a soil is 
simply the sum of .the surfaces of all the particles. It 
depends upon the shape and size of the particles. 

The surface l of a sphere of unit diameter is 31416 
square units. With spheres of smaller diameter there 
would be a greater number in a given volume, and the 
total surface would be increased. Thus, if the diameter 
were divided by a there would be a 3 times the number and 
the total surface would be a times 3'1416. In short, the 

1 The surface of a sphere is found by the formula s = 4 TT r 2 . 



THE PHYSICAL PEOPEETIES OF SOILS 55 

total surface of a mass of spheres is inversely proportional 
to the diameter. 1 

If the particles were as small as those of sand say T Jo 
of an inch the internal surface would be 3*1416 X 100 = 
314*16 square inches in 1 cubic inch, equal to 418*88 square 
yards in 1 cubic foot. 

' If the particles were as small as those of clay say ^"oo 
of an inch the internal surface would be 3*1416 X 5,000 = 
15,708 square inches in 1 cubic inch, equal to 20944 square 
yards in 1 cubic foot. 

According to this calculation, the internal surface of a 
cubic foot of clay is equal to a superficial area of over four 
acres. In clay soils, as a rule, only some 30 or 40 per cent, 
of the particles are as small as has been assumed above, 
but, on the other hand, some of them are smaller. The 
estimated amount of internal surface in a cubic foot of 
different kinds of soil is approximately as follows : 

Square feet. 

Sandy soils 5,000 

Clay soils . 100,000 

Calcareous soils 200,000 

Humous soils 500,000 

Mass. The mass of a body means the quantity of matter 
in it. It depends upon the density and volume. M=Vd. 
Compared with water as unity, the density of most of the 
rock-forming minerals lies between 2*5 and 3*5, but 
magnetite, haematite and some others are heavier about 
5*1. The density of the commoner rocks varies from about 2*5 
to 3*0 ; that of clay and sand have already been given as 
2*5 and 2*62 respectively, and that of peat as from 1*3 to 
1*6. Humus, therefore, is the only constituent that affects 
the density of soils to any considerable extent, and, with, 






56 



SOILS AND MANUEES 



the exception of vegetable soils, they do not differ much in 
this respect. The true specific gravity is usually about 
2*5 or 2*6. The mass of a given volume of soil, however, 
cannot be determined from the density of the particles 
because, in pulverent material, the space is not completely 
occupied by them. 

The proportion of unoccupied space in soils varies con- 
siderably, and the masses of equal volumes are therefore 
often very different. The weight of a given volume of soil 
divided by the weight of an equal volume of water is called 
the apparent specific gravity. 

The volume weight and apparent specific gravity of some 
soils and soil constituents are as follows : 






Volume Weight. 
Lbs. per cubic foot. 


Apparent Specific 
Gravity. 


Water . ... 


62-32 


1-000 


Quartz sand 


90-3 


1-449 


Clay. . ... 


63-0 


1-011 


Humus . ... 


20-9 


0-335 


Arable soil 


75-4 


1-206 


Old pasture soil 


65-5 


1-048 


Stiff land, 100 years in grass . 


59-1 


0-946 



The difference between the apparent specific gravities of 
sand and clay is greater than the difference between their 
true specific gravities, because there is more unoccupied 
space in the latter. In the case of the three samples of soil 
the differences are due partly to the same cause and partly 
to the larger proportions of humus which some of them 
contain. The presence of stones increases the volume 
weight of a soil because, being compact and impenetrable, 
the apparent specific gravity of stones is the same as the 
true specific gravity. The low r er layers of soil have, as a 
rule, a higher apparent specific gravity than the upper 



THE PHYSICAL PEOPERTIES OF SOILS 



layers because they are more compact, contain less humus, 
and, often, more stones. In trials made at Eothamsted the 
following results were obtained : 






Arable Land. 


Old Pasture. 


Volume 
Weight. 


Apparent 
Specific 
Gravity. 


Volume 
Weight. 


Apparent 
Specific 
Gravity. 


Top layer, 9 in. deep . 
Second layer, 9 in. 18 in. deep 
Third layer, 18 in. 27 in. deep . 
Fourth layer, 27 in. 36 in. deep 


89-4 
93-2 
98-4 
101-4 


1-434 
1-495 
1-579 
1-627 


71-3 

94-8 
100-2 
102-3 


1-144 
1-521 
1-607 
1-642 



Calculating from the volume weights given above, the 
mass of an acre of sand 9 inches deep would be 
2,950,101 Ibs., of clay, 2,058,210 Ibs. In the case of the 
old pasture land at Eothamsted the mass of the first 
9 inches would be 2,328,973 Ibs., and of the fourth 
layer 3,343,737 Ibs. per acre. In round figures the mass 
of an acre of land 9 inches deep may be from two to 
three million pounds. 

MOISTURE. 

Importance of Soil Moisture. One of the most impor- 
tant functions of soils is to supply water to the crops. 
Fertility depends upon their capacity to adequately per- 
form this, without interference with other functions, 
perhaps more largely than upon any other single condition. 
It is well known to farmers that a wet or a dry spot in a 
field makes more difference to the crops than any manure 
or treatment that can be applied to it. The capacity of 
the soil to provide the necessary water depends upon the 
climate and the physical properties mainly the size and 
arrangement of the particles of the soil. 



58 SOILS AND MANDEES 

Sources of Soil Moisture. The moisture of soils is 
derived principally from the rain, snow and dew precipi- 
tated upon them, but they also gain some by means of their 
hygroscopic and deliquescent properties. Soils lose water 
by percolation and evaporation, but some is retained and 
capillary phenomena cause it to be transferred from one 
place to another. The amount of water precipitated 
depends wholly, and the amount evaporated partly, on the 
climatic conditions. The gain and loss of water in other 
ways depends chiefly upon the physical properties of the soil, 
though, in a sense, they are affected by the climate too. 

Precipitation. In this country the mean annual rain- 
fall varies from about 25 to 50 inches, but in some 
localities it is much more and in others less. One inch 
of rain is equal to about 4*7 gallons per square yard, or 
over 100 tons per acre. Much of the water precipitated 
in the form of snow never penetrates the soil at all ; it 
melts at the surface and runs off. A much larger amount 
of dew is formed on the herbage than on the soil itself ; 
what does fall on the soil probably remains on the surface 
and is soon evaporated. 

Hygroscopy. Dew is formed only in a saturated atmo- 
sphere, but soils can also absorb water vapour from a non- 
saturated atmosphere by hygroscopic and deliquescent 
action. The former is a purely mechanical process, quite 
independent of the nature of the soil or its constituents. 
All gases exhibit a tendency to become denser when in 
contact with solid surfaces, and are therefore absorbed 
in large quantity by porous bodies which present a great 
extent of surface in small volume. The bleaching of 
organic colouring matters by charcoal and the ignition of 
hydrogen jets by spongy platinum are familiar illustra- 
tions of this phenomenon. Soils act in a similar manner, 
especially those of finer texture. In virtue of their great 
internal surface they possess in a high degree the power 



THE PHYSICAL PEOPEETIES OF SOILS 59 

of absorbing gases, and easily condensed vapours like 
water are reduced to the liquid state. The amount of 
water soils can absorb in this way, however, depends partly 
on the degree of saturation of the atmosphere as well as 
upon the extent of the internal surface. 

Deliquescence. The property known as deliquescence is 
due to the presence in the soil of certain substances which 
possess the power of attracting water vapour from the air 
independently of their physical condition. The water 
probably combines with the substances to form hydrates, 
as it is not completely evaporated on exposure to dry air, 
even at a temperature of 100 C. Calcium chloride and 
phosphoric acid are examples of highly deliquescent bodies, 
out common salt, hydrates of iron, alumina and silica and 
other substances commonly present in soils, exhibit similar 
properties in different degrees. 

The power pf a soil to absorb water vapour from the 
air is determined either by drying the soil at 100 C. and 
then exposing it to a saturated atmosphere, or by first 
exposing it to a saturated atmosphere and then drying at 
100 C., but the two methods give different results. The 
experiments make no distinction between hygroscopic and 
deliquescent moisture, and are probably not of much value 
as an indication of the properties of the undried soil. All 
purely hygroscopic water evaporates on exposure to dry air 
at ordinary temperatures, but the moisture attracted by 
deliquescent bodies is at least partly retained by them. It 
seems unlikely that water which requires heat to expel it 
from the soil can ever be of much benefit to plants. 

The absorptive power of pure sand is practically nil; 
the grains are too large for hygroscopic action and it con- 
tains no deliquescent bodies. Pipeclay, owing to its finer 
texture, can absorb about 9 per cent, of water, and clay 
soils, containing a large proportion of deliquescent col- 
loidal hydrates, much more. 



60 SOILS AND MANURES 

Capacity for Water. The capacity of a soil for water 
means the amount of water required to completely saturate 
it, i.e., to fill up all the interspace. It can be measured 
by direct experiment, but is more accurately determined 
by calculation of the interstitial space (p. 54). 

The capacity of different kinds of soil for water is 
approximately as follows : 

Per cent. 

Sandy soils (about) 40 

Loamy ,, x ....... 50 

Clay 60 

Humous 7080 

The capacity for water of an arable and an old pasture 
soil from the same locality was found to be 53 and 64 per 
cent, respectively. The difference was attributed to the 
larger proportion of humus in the latter. The capacity for 
water can be materially increased by addition of organic 
matter to the soil. 

Retention of Water. Under natural conditions soils do 
not long continue in a saturated state. A portion of the 
water percolates downwards under the influence of gravity, 
but some is retained. The retention of water by soils can 
be explained by reference to the phenomena of surface 
tension which cause suspended particles of liquids to 
shrink to the spherical shape and exert a pressure towards 
the centre. When a soil is moistened with water each 
particle becomes enclosed in a film which forms an 
elastic envelope about the particle, and is held in place by 
the pressure due to the surface tension of the liquid. The 
smaller the quantity of water the thinner will be the films 
and the greater the pressure. When the quantity of water 
is larger the films become thicker, the pressure is 
relaxed, and some lodges in the interstices. But the force 
of gravity increases in proportion to the mass of the water, 
and a point is ultimately reached at which it is equal to 



THE PHYSICAL PEOPEETIES OF SOILS 61 

that of surface tension. Any excess of water beyond this 
amount is drawn downwards and a state of equilibrium 
between the two forces is again established. 

The amount of water which a soil can retain depends 
on the size and approximation of the particles. The 
smaller particles present a larger internal surface to be 
covered with films of water, their points of contact are 
more numerous and the interstices are smaller. If the 
particles are very small the interstices remain full and the 
soil continues saturated to the limit of its capacity. When 
the particles are larger the force of gravity overcomes that 
of surface tension and the interstices are emptied, but the 
films covering the particles remain. 

Mayer found that quartz sand consisting of approxi- 
mately uniform particles of less than 0*3 m.m. diameter 
retained practically the whole of the water required to 
saturate it. 

Schloesing tested different kinds of soil and found the 
following quantities of water retained per 100 parts of 
dry soil: 

Kind of Soil. Water Retained. 
Coarse sand ....... 3 per cent. 

Fine sand ........ 7 ,, 

Calcareous sand 32 ,, 

Clay soil 35 

Forest soil ....... 42 ,, 

In general the water-holding power of sandy soils is too 
small and that of clays too great, but in both cases it can be 
modified by treatment. The retentive power of sands is 
sensibly increased by rolling, and that of clays is reduced 
by pulverising and loosening. Consolidation diminishes 
the size of the interspaces and increases the number of 
points of contact, while pulverising has the opposite effect. 
The water-holding power of sands can also be increased 



62 SOILS AND MANUKES 

by mixing with them substances of greater retentive power 
such as clay, lime and organic matter. 

If the soil contain too much water, air is excluded and 
the health of the plant Buffers ; if it contain too little, 
growth is retarded. Hellriegel and Wollny concluded, from 
certain experiments on this subject, that, in general, the 
best results are obtained when the soil contains from 40 to 
60 per cent, of the water required for complete saturation, 
i.e., when about half the interspace is occupied by water 
and the remainder by air. 

Coarse sandy soils, however, do not naturally retain more 
than about 10 per cent, of the water required to saturate 
them, but on the other hand they yield it up to the plants 
much more freely than some other soils of greater reten- 
tive power. Heinrich estimated the amount of moisture 
left in different kinds of soil from which water had been 
withheld until the plants growing in them wilted, and he 
obtained the following results : 

Water left in 

Soil when 
Kind of Soil. Plants Wilted. 

Coarse sand 1-5 per cent. 

Sandy garden soil 4'6 ,, 

Fine humus sand ...... 6'2 ,, 

Sandy loam ....... 7'8 ,, 

Calcareous loam 9'8 ,, 

Peat ... . 49-7 

The beneficial effects of farmyard manure on light land 
are generally attributed, at least in part, to the fact that 
it increases the water-retaining power of the soil. It may 
be questioned whether the water so retained is ever of much 
benefit to the plants. Heinrich's experiment would appear 
to suggest that if there were any scarcity of water in the 
soil the organic matter would not yield up to the plants 
what it retains, but might even be in competition with 
them for what is left. Long experience, however, has con- 




THE PHYSICAL PKOPE&TIES OF SOILS 



63 



vinced farmers that this is not so, and numerous scientific 
experiments have shown that,when compared with artificial 
manures, farmyard manure shows to best advantage in 
dry seasons, i.e., under conditions least favourable to its 






JP IG> Q. Apparatus showing the Height to which Water 
will rise in Tubes of different Calibre. 

action. There is little room for doubt that the water 
retained by the humus of the soil is largely beneficial to 
plants. 

Capillarity. It has been shown that if the quantity of 
water in the soil exceeds a certain amount, the force of 
gravity overcomes that of surface tension and the excess 
is drawn downwards. If the soil contains less than that 



64 SOILS AND MANUKES 

amount, the reverse action takes place ; the force of 
surface tension overcomes that of gravity, water is drawn 
upwards, and the films become thicker until equilibrium 
is established. This phenomenon is called capillarity, 
because it resembles the rise of water in fine tubes (Fig. 8) 
and is due to the same cause. 

There is, of course, no free water surface underneath 
the soil, but excess percolates downwards and evaporation 
takes place from the surface. The lower layers of s-oil, 
therefore, generally contain more water than those above, 
and some of it is drawn upwards by capillary action. But 
if the upper layers contain more water than the lower as 
when rain falls on very dry soil the force of surface 
tension acts in the same direction as gravity and the water 
descends very rapidly under their combined influence. 
Equilibrium is constantly disturbed by the introduction 
and abstraction of water, and the water is constantly 
moving downwards or upwards according to the conditions. 
The surface tension of the water causes it to move from 
any point where there is more to where there is less. The 
movement takes place laterally as well as vertically, and 
tends to equalise the distribution of the water in the soil. 

The apparatus employed for the study of capillary 
phenomena in the laboratory consists simply of a series of 
glass tubes each about 60 inches long by 1 inch in 
diameter, and closed by a piece of linen tied over one end. 

The tubes are filled with dry soil and fixed in vertical 
position with the closed ends immersed in water (Fig. 9). 

From observations made in this way it has been con- 
cluded that capillary action depends mainly upon the size 
and arrangement of the particles. In general, the smaller 
the particles and the more numerous the points of contact, 
the greater the height to which the water will rise and the 
greater the quantity of water which will rise to a given 
height. The rise takes place, at first, more slowly in fine 



THE PHYSICAL PKOPERTIES OF SOILS 



65 



than in coarse material, owing to the greater resistance in 
the former. In all cases, the rate at which the water rises 
falls off, and the quantity diminishes as the height 




FIG. 9. 



S.M. 



66 SOILS AND MANURES 

increases. Finally the rate becomes very slow and the 
quantity very small. 

The maximum height to which water will rise in clay 
soils is about 4 feet. In very fine sandy soils the water 
rises to nearly the same height and very much faster. The 
presence of colloidal hydrates in clay tends to block the 
passage of the water. In soils of coarser texture the water 
rises faster, but riot so high, and the quantity raised to 
any given height is much smaller. The presence of humus 
in the soil greatly increases its capillary power. 

In estimating the importance of capillarity as a means 
of bringing moisture to the upper soil, not only the charac- 
ter of the material, but also the supply of underground 
water must be taken into account. When the soil is 
saturated at a depth not exceeding the range of its capil- 
lary action, a large quantity may be raised to the surface 
in this way. This condition is, however, of very rare occur- 
rence, and it is probable that, under ordinary circum- 
stances, the quantity of water raised to the surface is not 
very great. Boots, however, penetrate into the soil, in 
some cases to a considerable depth, and water may be brought 
within their reach by capillary action, though it cannot 
bo raised to the surface. 

Percolation. Under ordinary conditions the force of 
surface tension is opposed to that of gravity, and the rate 
of percolation is therefore diminished by the same cir- 
cumstances that increase the power of retention, viz., the 
smallness and closeness of the particles. When more 
water than the soil can retain accumulates in the neigh- 
bourhood of the drains, the excess passes into them and is 
discharged. The rate of percolation is measured by the 
quantity of water that drains from a saturated soil in a 
given time. In gravels and coarse sands it is extremely 
rapid and very little water is retained. The opposite 
extreme is found in soils of finest texture in which the 



THE PHYSICAL PEOPERTIES OF SOILS 67 

whole of the water is retained; no drainage takes place 
at all, and the soil remains completely saturated. The 
permanence of either of these conditions in the surface 
soil would of course entirely preclude fertility, but the 
rate of percolation in cultivated land may approach one 
or other more or less closely. The naturally great resist- 
ance of fine materials is increased in clays by the presence 
of colloidal hydrates, which tend to block the passage of 
the water below and to hinder the entrance of air above. 
A shower of rain often temporarily arrests the flow of 
the drainage by blocking the pores at the surface. A 
contrary effect may be produced by expansion of the air 
in the soil, consequent upon a diminution of the barometric 
pressure or a rise of temperature. 

The amount of the drainage depends upon the rainfall. 
The records which have been carefully kept at Kothamsted 
since 1870 afford reliable information regarding the rela- 
tion between the two at that place. The gauges by means 
of which the percolation is measured consist of blocks 
of natural soil in situ. They were constructed by under- 
mining the soil at three different depths 20, 40 and 60 
inches respectively and inserting perforated iron plates 
to support it. This done, trenches were cut round the 
blocks of soil and these were then isolated by means of 
brick and cement walls. The soil taken out of the trenches 
was then returned to the spaces outside the walls. The 
percolating water is received in large zinc funnels, from 
which it passes into the measuring cylinders. The sur- 
face area of each gauge is exactly y^o part of an acre. 
The soil is kept entirely free from weeds and other growths, 
and the figures given in the table on p. 68 represent the 
drainage from bare soils. 

The rainfall is measured by means of .a similar gauge 
consisting of a rectangular zinc tray of the same surface 
area jcfco acre constructed close to the drain gauges. 

F 2 



68 



SOILS AND MANUEES 



An ordinary 5-inch funnel gauge is also employed. It gives 
a slightly lower result, the difference being J inch to 1 inch. 

MONTHLY KAINFALL AND DRAINAGE AT KOTHAMSTED. 
Average of 34 years, 18711904. 





Rainfall 


Percolation through Soil. 




r&s acre. 






Gauge. 


20 in. 


40 in. 


60 in. 


Per cent, 
of Rainfall. 




Inches. 


Inches. 


Inches-. Inches. 


Per cent. 


January . 


2-32 


1-82 


2-05 


1-96 


78-4 


February 


1-97 


1-42 


1-57 1-48 


72-1 


March 


1-83 


0-87 


1-02 


0-95 


47-5 


April 


1-89 


0-50 


0-57 


0-53 


26-4 


May 


2-11 


0-49 


0-55 


0-0 


23-2 


June 


2-36 


0-63 


0-65 


0-62 


26-7 


July . 


2-73 


0-69 


0-70 


0-65 


25-3 


August . 


2-67 


0-62 


0-62 0-58 


23-2 


September 


2-52 


0-88 


0-83 


0-76 


34-5 


October . 


3-20 


1-85 


1-84 


1-68 


57-8 


November 


2-86 


2-11 


2-18 


2-04 


73-8 


December 


2-52 


2-02 


2-15 


2-04 


80-1 


Whole Year . 


28-98 


13-90 


14-73 


13-79 


47'9 



Results for Maximum and Minimum Rainfall. 



Maximum, 1903 
Minimum, 1898 


38-69 
20-49 


23-48 
7-32 


23-60 
7-90 


24-23 
7-69 


60-7 
35-7 



It will be seen that the three gauges give practically 
the same results both for the average of the whole period 
and in the two abnormal years of maximum and minimum 
rainfall. 

On the average of the thirty-four years the rainfall was 
nearly 29 inches, and, of that quantity, nearly one half 
passed into the drains. In 1903, with a rainfall of over 



THE PHYSICAL PEOPEETIES OF SOILS 



69 



38 inches, over 60 per cent, escaped in the drainage, but in 
1898, when the rainfall was just over 20 inches, only about 
35 per cent, escaped in the drainage. 

Evaporation. During the six winter months October 
to March the total rainfall amounted to 14*7 inches, and 
during the six summer months April to September it 
was 14'28 inches, or very nearly the same. The amount 
of drainage during these two periods was, however, very 
different, being 10*09 inches during the winter and only 
3'81 inches in the summer. The difference is due to the 
greater amount of evaporation which takes place during 
the warm summer weather. 

The following table shows the average monthly evapora- 
tion, i.e., the difference between the rainfall and the 
drainage : 

MONTHLY KAINFALL AND EVAPORATION FROM BARE SOIL AT 
EOTHAMSTED. 

Average of 34 years, 18711904. 





Rainfall 
1 acre 


Evaporation from Soil. 




Gauge. 


20 in. deep. 


40 in. deep. 


60 in. deep. 


Per cent, 
of Rainfall. 




Inches. 


Inches. 


Inches. 


Inches. 


Per cent. 


January . 


2-32 


050 


0-27 


0-36 


21-6 


February 


1-97 


0-55 


0-40 


0-49 


27-9 


March 


1-83 


0-96 


0-81 


0-88 


52-5 


April 


1-89 


1-39 


1-32 


1-36 


73-6 


May 


211 


1-62 


1-56 


1-61 


76-8 


June 


2-36 


1-73 


1-71 


1-74 


73-3 


July 


2-73 


2-04 


2-03 


2-08 


74-7 


August . 


2-67 


2-05 


2-05 


2-09 


76-8 


September 


2-52 


1-64 


1-69 


1-76 


65-4 


October . 


3-20 


1-35 


1-36 


1-52 


42-2 


November 


2-86 


0-75 


0-68 


0-82 


26-2 


December 


252 


0-50 


0-37 


0-48 


19-9 


Whole year 


28-98 


15-08 


14-25 


15-19 


52-1 



70 SOILS AND MANUEES 

Results for Maximum and Minimum Rainfall. 



Maximum, 1903 
Minimum, 1898 


38-69 
20-49 


15-21 
13-17 


15-09 
12-59 


14-46 
12-80 


39-3 
64-3 



For convenience of interpretation the figures given in 
the table may be arranged in quarterly periods as 
follows : 



INCHES OF WATER EVAPORATED. 



Oct., 1-35 
Nov., 0-75 
Dec., 0-50 

2-60 



Jan., 0-50 
Feb., 0-55 
Mar., 0-96 

2-01 



April, 1-39 
May, 1-62 
June, 1'73 

4-74 



July, 2-04 
Aug., 2-05 
Sept., 1-64 

5-73 






Rainfall. 


Drainage. 


Evaporated. 




Indies. 


Inches. 


Per cent. 


Inches. 


Per cent. 


Six months, Oct. March . 
,, April Sept. . 


14-70 
14-28 


10-09 
3-81 


68- 6 
26- 7 


4-61 
10-47 


31- 4 
73- 3 


Whole year 


28-98 


13-90 


47-96 


15-08 


52-03 



It will be seen that the amount of water evaporated was 
lowest 0*5 inch in January, and gradually rose, month 
by month, to a maximum of 2*05 inches in August. After 
that it gradually declined again to the minimum of 0'5 
inch in December. The amount of evaporation is prac- 
tically the same in the months of July and August, May 
and September, December and January. During the six 
summer months the rainfall was nearly the same as in the 
six winter months but the ,amount evaporated was more 
than double. 

When annual periods are considered a striking constancy 








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>72 SOILS AND MANURES 

is noticeable in the amount of water evaporated from year 
to year notwithstanding considerable variations in the rain- 
fall. The lowest rainfall 20'49 inches was recorded 
in 1898 and the highest 38'69 inches in 1903. The 
latter is nearly double the former, the difference between 
them being 18.2 inches, but the difference in the amount 
evaporated is only about 2 inches. 

As the soil dries at the surface, water rises from below 
by capillary action and the soil dries up to a consider- 
able depth. Anything which interferes with capillary 
action, therefore, must tend to check evaporation, and so 
to conserve the moisture in the soil in periods of drought 
when it is most required. The most obvious and one of the 
most effective methods is to cover the soil over with a mulch 
of straw or other material into which the water cannot rise 
by capillarity. This is not always practicable, and it 
interferes with cultivation. But light surface cultivation 
itself hoeing, scuffling, etc. has a similar effect. It 
breaks the capillary connection; the loose soil at the sur- 
face becomes very dry and hot, but forms a protective 
covering which, like the mulch, checks evaporation. Many 
gardeners believe that, as a means of keeping the soil 
moist, the Dutch hoe is better than the ^watering can. 
Deeper cultivation promotes evaporation by opening up the 
soil to the circulation of dry air which carries off the mois- 
ture. Consolidation of the surface soil also tends to pro- 
mote evaporation by increasing the capillary action. 

The loss of water by evaporation from soils under crop is 
enormously greater than from bare soils. The latter alone 
has, so far, been under consideration. The amount of 
water absorbed from the soil by large trees is so great that 
the ground underneath them is often quite bare, notwith- 
standing the reduced evaporation from the surface due to 
the shade they afford. A tree planted in a hedgerow 
almost invariably ruins the hedge for some distance on 



THE PHYSICAL PROPERTIES OF SOILS 



either side by depriving it of the necessary moisture. 
Plants take up their food from the soil in very dilute solu- 
tions, and, in order to obtain sufficient, a large amount of 
water must be transpired. 

Numerous attempts have been made to estimate the 
amount of water transpired by crops during their period of 
growth. The methods have been generally alike and the 
results have been given as pounds of water transpired for 
each pound of organic matter formed. ,The results obtained 
by different observers do not agree very closely, but it is 
evident that the amount of water evaporated depends upon 
the kind of plant and the amount of water and of available 
plant foods in the soil. Some idea of the amount of water 
transpired by plants may be formed from the following 
figures : 



Observers. 


Pounds of Water Transpired for each Pound 
of Organic Matter formed. 






Mean, 


Lawes and Gilbert 


214262 


238 


Hellriegel ..... 


262402 ~ 


332 


King 
Wollny 


272774 
233912 


523 

572 


Mean ..... 


245587 - 


416 



It will be seen that on the average the crops transpire 
about 400 times their dry weight of water. A crop which 
yields 4,000 Ibs. of dry matter per acre at harvest would 
thus transpire about 1,600,000 Ibs. of water, or more than 
7 inches 1 of rain, during its period of growth. The quantity 
of dry matter in a crop of meadow hay is generally less 
and in a turnip crop more than that given above. Cal- 

1 Taking the mass of the soil at 70 pounds per cubic foot or three 
million pounds per acre 12 inches deep, 1 per cent, of moisture is 
equal to about 13'39 tons of water per acre or 0'133 inches of rain. 



74 SOILS AND MANURES 

culated on the same basis, the quantity of water transpired 
by the former is equal to about 5 inches of rain and by the 
latter to about 8 inches. 

TEMPERATURE. 

The temperature of the soil has an important influence 
on fertility. Not only does it affect the crops directly at 
all stages of growth, but it also has a great effect upon the 
chemical and biological changes within the soil, upon which 
fertility largely depends. The vital activity of most vege- 
table organisms is entirely arrested at temperatures below 
the freezing point of water and above 50 C. The most 
favourable temperature is generally above the mean of 
these two, and considerably higher than the mean tempera- 
tures commonly recorded in this climate. Anything which 
tends to raise the temperature of the soil will therefore, as 
a rule, increase the fertility, and anything which tends 
to lower it will have a contrary effect. 

The great source of heat in the soil is, of course, the sun, 
which pours its rays upon the earth. Physical and chemi- 
cal changes within the soil contribute a certain amount, 
but experiments have shown that, under ordinary circum- 
stances, this heat has a very small effect on the tempera- 
ture. Some heat is also derived from the interior of the 
earth ; the amount is unknown but it may be considerable. 
In all probability it is constant throughout the year and 
varies but little from place to place. 

The climate of a place depends mainly upon the situa- 
tion latitude, altitude and aspect the character and 
direction of ocean currents, distance from the sea, direction 
of prevailing winds, height and direction of mountain 
ranges, and afforestation. 

Influence of Latitude. Any portion of the surface of 
the earth receives most heat from the sun when the rays 



THE PHYSICAL PROPERTIES OF SOILS 75 

fall vertically upon it, i.e., when the plane of the surface 
makes a right angle with the direction of the rays. Only 
a comparatively narrow zone, lying between the parallels 
of 23J north and south latitude, ever occupy that most 
favourable position ; in every other latitude the sun's rays 
always strike the earth at an angle which is more or less 




FIG. 11. 

acute according to the time of year. For this reason, and 
also because more heat is absorbed by the atmosphere, 
owing to the greater depth traversed by the sun's rays, 
in the higher latitudes the soil receives less heat from the 
sun. 

These effects are illustrated in the diagram (Fig. 11). 
First, if the divisions of the straight line A B, B C, C D, 
etc., represent equal amounts of sunshine, the corre- 
sponding divisions a b, b c, c d, etc., will represent the 
extent of the portions of the surface of the earth which 



76 SOILS AND MANUKES 

receive them. It will be seen that 'in the higher latitudes 
equal amounts of sunshine are spread over much larger 
areas and have much smaller calorific effect per unit of 
surface. Second, if the circle K T represent the outer 
limit of the atmospheric envelope, the distances a' a, b f I, 
c r c, etc., represent the depth of air traversed by the rays, 
and it will be seen that they are much greater in the higher 
latitudes. 

Influence of Altitude. The higher mountain peaks and 
more elevated positions probably receive less heat from 
the interior of the earth. The air, in the higher regions, 
contains less water vapour and is less dense ; it has, there- 
fore, for equal volumes, a much smaller capacity for heat. 
Also it receives less heat by radiation from the surface of 
the earth. The influence of altitude on the climate may 
be seen from the fact that the line of perpetual snow is 
found 

In latitude (equator) . . 18,000 feet above sea level. 

46 (Geneva) . . 9,000 

60 (St. Petersburg) . 4,000 . 

,, ,, 75 (Nova Zembla) . at the sea level. 

Influence of Aspect. For the reasons given above, in 
the higher northern latitudes the soil receives more heat 
from the sun when it has a southern slope or aspect. This 
may be seen from the diagram (Fig. 12), in which A B C D 
represents a sunbeam falling upon a flat portion of the 
surface of the earth A D at an angle of 30. If A D be 
horizontal, the sunshine falling upon it is represented by 
the line B C ; but if A D be inclined to the northward at 
an angle of 10 as e D the sunshine falling upon it will 
be reduced to E C ; if the angle of inclination be 20 as 
/ D the sunshine will be reduced to F C. If A D be 
inclined to the southward at an angle of 10 as h D the 
sunshine falling upon it will be increased to H C ; if the 



THE PHYSICAL PROPERTIES OF SOILS 



77 



angle of inclination be increased to 20 as i D the sun- 
shine will be increased to I C. The greatest amount of 




FIG. 12. 



heat is received by the soil when the land slopes at an 
angle corresponding to that at which the sun's rays strike 



78 SOILS AND MANUEES 

the earth, i.e., when A D is parallel to B C and makes a 
right angle with the direction of the rays. In the figure 
this position of maximum is an angle of 30, because that 
is the angle at which B C is drawn. 

Influence of Colour. Dark soils absorb more heat from 
the sun's rays than those of lighter colour. Experiments 
have shown that considerable difference of temperature 
may be produced by artificially darkening or lightening the 
surface of the soil. The effects are only noticeable in 
bright sunshine, and are probably much greater in the 
tropics than in more temperate climates. The darker 
shades of colour are due mainly to the organic matter. 
Some of the prairie soils in North America which contain 
a large proportion of this constituent are quite black, and 
it is stated that, on this account, these soils are more suit- 
able for the production of maize, and crops which require a 
large amount of heat, than lighter coloured soils situated 
much further south. Under natural conditions the thermal 
effects of colour are probably not of great practical impor- 
tance in this country. 

Capacity for Heat. The capacity for heat ,or specific 
heat of a body means the amount of heat jt v can hold or 
store up in a definite quantity of the substance. It is 
measured by the extent to which the temperature of the 
body is raised by a given amount of heat. For example, 
a given amount of heat will raise the temperature of a 
pound of quartz five times as much as that of a pound of 
water; or it will raise the temperature of five times as 
much quartz to an equal extent. The capacity for heat 
of quartz is therefore one -fifth of that of water. The 
capacity for heat of water is greater than that of any other 
substance and is taken as unity. 

The mineral constituents of the soil all exhibit much 
the same capacity for heat about 0'2 or 0'25 but that of 
organic matter is greater, viz., about 0*5 (water = 1). 



THE PHYSICAL PEOPEETIES OF SOILS 79 

The presence of organic matter in the soil therefore 
increases the capacity for heat, but, as it rarely amounts 
to more than 10 per cent, of the dry matter, except in soils 
of vegetable origin, its effect is not great. 

The capacity for heat of the dry matter of soils is usually 
about 0*2, and any variation, due to different proportions 
of organic matter or of the several mineral constituents, is, 
as a rule, too small to sensibly affect the temperature of 
the soil. 

Under natural conditions the capacity for heat of soils 
depends almost entirely upon the quantity of water they 
contain, and the variations due to this cause have an im- 
portant effect on the temperature. For example, if a cubic 
foot of soil contain 75 Ibs. solids 1 and 25 Ibs. water, and 
absorb enough heat to raise the temperature 5, the same 
amount of heat applied to the dry matter alone would have 
raised the temperature 12'9. The water absorbs some of 
the heat and the temperature is thereby reduced 7*9. Again, 
equal amounts of heat would raise the temperature of 

75 Ibs. dry soil 10 C. 

75 " +10 Ibs. water .... 6'1 C. 
75 ,, + 20 . . . . . 4-4 C. 
75 +30 . . 3-4 C. 

In the last case the reduction in temperature due to the 
presence of water is over 60 per cent. 

Soils which retain the smallest quantity of water coarse 
sands have the lowest capacity for heat and attain the 
highest temperature. Those which retain most water- 
humous soils have the highest capacity for heat and are 
the coldest. The capacity for heat of humus is greater 
than that of the minerals, but owing to the difference in 
density the capacity for heat of equal volumes is about 
the same. 

1 Specific heat 0'21. 



80 SOILS AND MANURES 

The following table shows the capacity for heat of equal 
masses of soils and soil constituents : 

Water . I'OO Loamy soil (dry) . . . . -21 

Humus . '48 same with 10% organic matter . -24 

Clay . . '23 Loamy soil 10% water . . . -29 

Limestone. '21 ,, ,, 20% ,, -37 

Quartz . -19 30% ,, -45 

Ferric oxide *16 

The capacity for heat of equal masses multiplied by the 
density gives the capacity for heat of equal volumes. 

Effect of Evaporation of Water on the Temperature of 
Soils. Some of the heat absorbed by the soil is dissipated 
by the evaporation of water. During the summer months 
the daily evaporation may amount to about a quarter of a 
pound or more of water per square foot of surface. The 
latent heat of water, i.e., the heat of vaporisation, at the 
ordinary temperature, is 588 units. The evaporation of 
a quarter of a pound of water would therefore absorb 
enough heat to reduce the temperature of 147 Ibs. of water 
1 C. If this amount of heat were distributed over one 
cubic foot of soil containing 75 Ibs. of solid matter and 
25 Ibs. of water the cooling effect would be equal to a 
reduction of temperature of 3'6 C. If the soil contained 
less water the cooling effect would be greater ; with half 
the quantity of water the cooling effect would be equal to 
a reduction of 4*5 C. of temperature. 

Evaporation generally takes place most rapidly when the 
temperature of the soil is actually rising under the 
influence of the sun's rays, but if the evaporation were 
prevented the increase of temperature would be so much 
greater. Any process which hinders evaporation, such as 
loosening the surface or covering it with manure, etc., 
tends to conserve the heat and sustain the temperature. 

Radiation of Heat. Soils cool down at night by radiat- 
ing heat from the surface, The speed of radiation depends 



THE PHYSICAL PROPERTIES OF SOILS 81 

chiefly upon the difference between the temperature of the 
air and that of the soil. It is also affected by the moisture 
and conductivity but not by the colour of the soil. 

Water radiates heat faster than the solid constituents, 
and its presence therefore increases the radiating power 
of the soil. As it cools at the surface the warmer water 
underneath tends to rise by convection. This helps to 
maintain the temperature at the surface but increases the 
total loss of heat. The heat brought to the surface by 
conduction has a similar effect. On the other hand, owing 
to its greater capacity for heat, the moisture of the soil 
diminishes the rate of cooling. 

Conduction of Heat in Soil. -- The power of soils to 
transmit heat by conduction is small. Solid compact rocks 
conduct heat about four or five times faster than water 
but not nearly so fast as metallic substances. When the 
rocks are broken down into a loose, pulverent condition, 
as in soils, the conductivity is greatly reduced, because 
the interspaces are filled with air of which the conductive 
power is less than a hundredth of that of the rock sub- 
stance. When the air is wholly or partially displaced 
from the interspaces by water the conductivity is increased 
because the water conducts heat more than twenty-two 
times faster than air. The conductivity of quartz is slightly 
greater than that of ,the other solid constituents of the 
soil, but there is very little difference between them in this 
respect. The presence of stones and large particles in the 
soil favours conductivity. Compression diminishes the size 
of the interspaces and increases the number of points of 
contact and therefore increases the conductivity. It is by 
conduction that heat penetrates into the soil. Dry pul- 
verent materials become hotter at the surface but remain 
colder underneath. The conductivity of the soil tends to 
equalise the temperature. 

The amount of heat developed by the condensation of 

S.M. G 



82 SOILS AND MANUKES 

water vapour by the hygroscopic and deliquescent action 
of the soil is too small to be of any practical importance. 

The oxidation of organic matter as a source of heat has 
already been briefly referred to (p. 11). A heap of fresh 
stable manure is usually warm and steamy, and when 
'partially dried by its own heat may become so hot as to 
catch fire. It is possible therefore to obtain almost any 
temperature desired by this means. In the hotbeds for 
forcing plants in gardens, layers 2 or 3 feet deep, or even 
more, are often used simply for the purpose of generating 
heat. This method is generally preferred to hot pipes or 
other devices because it heats the soil from below and has 
not the same tendency to dry the atmosphere and promote 
undue evaporation from the plants. The manure cannot 
bo profitably employed on the farm for this purpose all 
that is produced must be distributed over a much larger 
area to fertilise the soil but when large quantities 
are incorporated with the soil the temperature of the 
latter is sensibly elevated. The maximum rise of tem- 
perature due to admixtura of 10 tons of fresh manure was 
2 C., and this disappeared entirely in less than three 
weeks' time. Under the ordinary conditions of agriculture 
the effect of the heat produced by the oxidation of organic 
matter on the temperature of the soil is small and 
transient. 

The Temperature. The mean temperature of the soil 
at the surface generally follows that of the air very closely, 
but the range of temperature is smaller. Thus, when the 
monthly mean temperature of the air was 57'1 F. that of the 
soil was 56*7 F. The mean daily range of temperature of the 
air was 22 F., and that of the soil was only 15 F. Beneath 
the surface the daily range of temperature rapidly diminishes 
as the depth increases. At a depth of 6 inches the range 
of temperature was only 4'6F., and at 24 inches only 0'5 F. 
The daily variations generally cease to be noticeable at a 



THE PHYSICAL PKOPEETIES OF SOILS 



83 



depth of from 2 to 3 feet. The monthly and seasonal 
variations extend to a greater depth, but a point is 
ultimately reached at which these also are too small to 
be recorded. The depth to which the variations extend 
depends mainly upon the extent of the variations at the 
surface. 

During the winter months the temperature of the soil is 
generally rather higher than that of the air; in summer 
it is generally somewhat lower. The differences become 
more marked as the depth increases, as will be seen from 
the following figures : 






Temperature 
of Air. 


Temperature of Soil at Various Depths. 


1 inch. 


6 inches. 


12 inches. 


24 inches. 


February 
June . 


26-0 

68-7 


28-3 
66-4 


29-5 
65-6 


31-2 
54.70 


32-6 
61-8 



MISCELLANEOUS PROPERTIES. 

Colour. Of the fundamental substances which make up 
the bulk of the soil, organic matter alone is brown or black. 
Silica, kaolin and carbonate of lime, when perfectly pure, 
are white. Quartz crystals are often coloured by the pre- 
sence of traces of metallic oxides chiefly oxides of iron. 
Many of the common rock-forming minerals, e.g., felspars, 
etc., are white or greyish white in the powdered condition. 
Some of them, however especially those which contain 
compounds of iron in larger or smaller proportion are 
black or dark coloured. Magnetite and biotite are black ; 
hornblende is usually almost black with a greenish or some- 
times a reddish tinge ; augite and olivine are green ; 
haematite is red and pyrites a golden yellow colour. The 

G 2 



84 



SOILS AND MANURES 



various shades of colour seen in soils are due to the pre- 
sence of organic matter, coloured crystals and the admix- 
ture of black or coloured minerals in different propor- 
tions. The shades most commonly produced by the different 
combinations are as follows : 






Sand. 


Clay. 


Carbonate of Lime. 


Organic matter . 
Ferric oxide 
Ferrous oxide . 
Pyrites 


brown 
red 


black 
yellow to red 
blue 
green 


greyish black 
red 



Certain inferences may be drawn with regard to fertility 
from the colour of the soil, but they are by no means reli- 
able unless confirmed in other ways. The dark reddish 
brown to black shades are generally due to organic matter 
with more or less ferric oxide, and are regarded as a good 
sign. Ferric oxide alone produces various shades of yellow 
and red, and implies, at least, adequate oxidation. Blues 
and greens generally indicate the presence of sub-oxides 
of iron and possibly pyrites. These substances are poison- 
ous to vegetation, and their presence implies imperfect 
oxidation. These conditions are prejudicial to fertility, 
and all dark colours, except when due to organic matter, 
are to be regarded with suspicion. 

Odour. Soil, when moist, emits a well-known, distinc- 
tive odour, but none of the attempts to isolate the compound 
from which it arises have, as yet, met with success. The 
substance appears to originate in the organic matter, is 
volatile, of neutral reaction, and, when heated with potash, 
gives rise to ( a resinous product, but it does not exhibit 
other properties common to aldehydes. Until further 
knowledge is gained about the compound it cannot be 
regarded as of much importance. 



THE PHYSICAL PEOPEETIES OF SOILS 85 

Tenacity. In the moist condition the particles of soil 
exhibit a certain tendency to cohere together owing to the 
surface tension of the water. When the particles are very 
small, as in clay, the cohesive tendency is greatly inten- 
sified and produces that quality of " tenacity " which 
makes clay soils " heavy " to cultivate. The tenacity dimi- 
nishes as the size of the particles increases, and ultimately 
becomes imperceptible. " A rope of sand " is a proverbial 
expression to indicate lack of cohesion. Tenacity is greatly 
increased by the presence of gelatinous hydrates and other 
colloidal substances which colligate the particles. In the 
absence of such cementing agents the particles show but 
little tendency to cohere when dry. The most important 
cementing agents are colloidal clay, humic acid, and the 
gelatinous hydrates of iron, alumina and silica. 

It is well known that when a sample of strong clay is 
thoroughly disintegrated and shaken up with water the 
bulk of it settles to the bottom of the vessel on standing 
for a few hours but the water remains turbid for an inde- 
finite period. The solid matter which produces this tur- 
bidity is called colloidal or coagulable clay. It was at 
one time supposed that the chemical composition of this 
substance was essentially different from that of the par- 
ticles which remained suspended only for a few hours. 
There is some reason to believe that it may be perhaps more 
highly hydrated. This, however, is uncertain, and it is 
evident that it does not differ much in other respects. 
The extremely minute size of the particles of colloidal clay 
is sufficient to account for its cementing power and most 
of its other properties. 

The view has been expressed that the permanent sus- 
pension of colloidal clay marks only a lower limit of solu- 
tion. The phrase is ambiguous, and may be equally applied 
to all other colloid or gelatinous substances which, when 
they appear to be dissolved, are probably merely suspended 



86 SOILS AND MANURES 

in a state of infinitely fine division. To say that the 
colloidal state, at least so far as clay is concerned, marks 
the ultimate limit of division in solids would probably be 
nearer the truth. 

The addition of a few drops of lime water to the turbid 
liquid containing the colloidal clay in suspension causes 
the suspended particles to coalesce and form aggregates or 
flocules, which soon settle out, leaving the liquid perfectly 
clear. Mineral acids, acid salts, and some normal salts, 
e.g., common salt, have a similar effect. Precipitation 
does not appear to be due to combination with the reagent, 
as the latter remains in solution and the precipitate can 
be redisseminated through pure water. 

Mechanical force causes defloculation or destruction of 
the aggregates, and is applied in various ways at potteries 
and brickworks for this purpose. Defloculation can also 
be effected by puddling and kneading, vigorous agitation 
with water and by boiling. It is promoted by the presence 
of alkalis and of alkaline salts. : 

According to one hypothesis, the floculating agents 
lime, alum, common salt, etc. cause displacement of some 
of the loosely combined water of hydration, and floculation 
results merely from the tendency to shrink in volume, 
which all highly hydrated substances exhibit when water is 
abstracted. The " sailing out" of soaps, albuminoids and 
other substances of complex molecular structure, has also 
been attributed to abstraction of water, and it is possible 
that the floculation of colloidal clay may be analogous. 

Whatever the cause or mechanism of the process there is 
no room for doubt as to its great practical importance. 
In the defloculated or gelatinous condition colloidal clay 
greatly increases the tenacity and blocks up the pores of the 
soil, and clay soils cannot be successfully cultivated unless 
kept in the floculent state. It is for this reason that 
they must not be " worked " or trampled when wet, and 



THE PHYSICAL PROPEKTIES OF SOILS 87 

that alkaline substances, such as wood ashes, should not be 
applied to them. As the soil dries, the cementing power of 
the colloidal substance increases and forms hard brick- 
like lumps which are not easily disintegrated by tillage. The 
hard lumps of soil, however, retain the power of reabsorbing 
water, and, when moistened, resume the plastic condition. 

When the colloidal clay is kept in the floculent condition, 
the wet soil becomes much more porous and less tenacious ; 
also, the lumps formed when the soil dries are not nearly 
so hard and are easily reduced to that finely pul- 
verent condition known as tilth, which is essential in the 
seed beds. Lime possesses this floculating property in 
pre-eminent degree, and its well-known ameliorating effects 
on clay soils are largely due to this cause. According to 
Hilgard, the addition of J per cent, of lime to a sticky 
clay soil will almost entirely change its character in 
that respect. Much less than this will produce a marked 
improvement, and even the relatively small quantity in an 
ordinary dressing of basic slag may have a certain bene- 
ficial effect. Acid substances like superphosphate act to 
some extent in a similar manner. 

The cementing power of colloidal clay appears the more 
remarkable in view of the small quantity of it found even in 
soils of the most strongly plastic character. The propor- 
tion probably never exceeds .2 or 3 per cent., but its in- 
fluence on the tenacity is, of course, much greater when the 
other particles of the soil are also small. 

The colligating power of humic acid is probably much 
greater than that of the colloidal hydrates of alumina, 
silica, etc. Schloesing found that 1 per cent, of humic 
acid, in the form of freshly precipitated calcium humate, 
had the same cementing power as 11 per cent, of plastic 
clay. Like colloidal clay, all these substances produce a 
much greater degree of tenacity when they are mixed with 
smaller particles. They do not remain permanently su$- 



88 SOILS AND MANUBES 

pended in water, and are not floculated by reagents to the 
same extent as colloidal clay, if at all. On drying they 
shrink greatly in volume and lose their binding power. 
They cannot take up water again or resume the plastic state. 

Contraction. The shrinkage of colloidal matters on 
drying, already referred to, causes a considerable contrac- 
tion in the volume of the soil as a whole, and produces the 
cracked appearance commonly seen in periods of drought. 
The phenomenon does not occur in sands at all but only in 
soils which contain much clay or organic matter. Air 
finds access to the mass of the soil through the cracks 
which spread out in all directions, and provides a supply of 
oxygen of which these soils generally stand much in need. 
The soils swell up again when moistened and the cracks 
are soon obliterated. 

Diffusion. By the decomposition of minerals and 
organic matter in the soil, various salts and compounds 
are constantly passing into solution and are again with- 
drawn by the roots of plants. Small local differences of 
concentration produced in these ways are equalised by the 
movements of the water which holds the substances in 
solution, and also by the movement of the dissolved salts 
through the water known as diffusion. The rate of diffu- 
sion depends partly on the nature of the substance and 
partly on the difference of concentration. Crystallisable 
substances, such as salts, diffuse at different rates, but 
all of them much more rapidly than colloids. Crystalloids 
diffuse through colloidal jellies or colloidal membranes 
nearly as rapidly as through pure water, but one colloid 
does not diffuse through another. 

MECHANICAL ANALYSIS. 

The object of physical or mechanical analysis of soils is 
to ascertain the physical or mechanical properties. It has 



THE PHYSICAL PEOPEETIES OF SOILS 89 

been shown that the physical properties depend ultimately, 
to a large extent, upon the size of the particles and amount 
of the interspace. These therefore are the chief points to 
be determined. 

In order to estimate the interspace without disturbing 
the arrangement of the particles it is necessary to procure 
samples of the soil in situ. A simple and convenient plan 
is to use a metal box 6 inches square by 8 inches deep, 
i.e., having a total capacity of one-sixth of a cubic foot. 
The soil is first dug away from around a block of soil 
having a surface area somewhat larger than that of the box ; 
the latter is easily pressed down over the block till the 
bottom is level with the surface, and then, by inserting a 
spade underneath, it can be lifted up full of soil. The 
weight of the contents of the box multiplied by six gives 
the volume weight of the soil in pounds per cubic foot. 
With a little practice duplicates can be drawn in this way 
which approximate very closely in weight. 

After weighing, the sample may be taken from the box 
and dried. The weight 1 of dry matter divided by the 
weight of an equal volume 1 of water gives the apparent 
specific gravity, and this divided by the true density 
gives the volume occupied by the solid matter. The 
weight of water in the sample is found by estimating the 
loss on drying and the volume occupied by it is easily 
calculated. The remainder of the space, not occupied by 
solids and water, is empty, i.e., occupied only by air and 
gases. 

The relative amounts of space occupied by solids, water 

1 When the cubic foot is employed as unit volume, it will be found 
convenient to adopt the ounce avoirdupois as unit of mass. For 
practical purposes the weight of a cubic foot of water maybe taken as 
1,000 ounces (the actual weight is 996-458 ounces). The relation of 
ounces per cubic foot is, therefore, almost exactly the same as grams 
per cubic decimetre, 



90 SOILS AND MANUKES 

and gases respectively in two cultivated soils examined by 
the author were as follows : 

i. ii. 

Space Occupied. Space Occupied. 

Per cent. Per cent. 

Solids 32-3 39'7 

Water . . . . 37'3 \ 47-1 ] 6Q . 3 

Gases .... 30'4 J 18*2 J 

100-0 100-0 



In sample I. the water amounted to 55 per cent, of the 
total capacity of the soil (i.e., total interspace), and plenty 
of space was left for air. The physical properties of the 
soil were therefore, to this extent, excellent. 

Sample II. was a soil which had been for a long time 
under grass, and, though formerly fertile, had in recent 
years yielded very poor crops. Heavy dressings of farm- 
yard manure and various kinds and combinations of arti- 
ficial manure produced little or no effect, and chemical 
analysis showed that it was not deficient in plant foods. 
The explanation is to be found in the figures given above. 
Though not badly drained, the water amounted to over 78 
per cent, of the total capacity of the soil, and the air space 
was reduced to 13*2 per cent, of the total volume. This 
soil was a stiff clay, in which the colloidal matter had 
become defloculated and blocked up the pores. Super- 
ficially it presented the appearance of a homogeneous 
jelly. It is almost superfluous to add that the proportion 
of lime in this soil was exceedingly small. 

The retentive power for water can best be judged from 
the amount found in the soil, as above described, if 
the sample is taken soon after the discharge of drainage, 
following upon heavy rain, has ceased. Eesults obtained 
by saturating a sample of soil with water in the laboratory 
and estimating the quantity left in it after draining are 



THE PHYSICAL PROPERTIES OF SOILS 



91 



purely artificial because, 
even if the arrangement of 
the particles has not been 
disturbed in taking the 
sample, the process takes 
no account of the influence 
of the subsoil. 

In estimating the size of 
the particles the experiments 
are performed upon the air- 
dried sample. The larger 
particles are first separated 
by sieves. All mineral par- 
ticles too large to pass 
through circular holes of 
5 m.m. diameter are classi- 
fied as stones and what 
passes through is called fine 
earth. Whatever part of the 
fine earth is retained by a 
sieve, in which the holes are 
of 1 m.m. diameter, must be 
composed of particles of less 
than 5 m.m. and greater 
than 1 m.m. diameter. By 
using sieves having holes of 
different size, the coarser 
particles can all be separated 
and divided into groups of 
different degrees of fineness. 

For the separation of 
particles below 0*5 m.m. 
diameter recourse must be 
had to some process of 
elutriation. Many methods 




d 




FIG. 13. 



92 SOILS AND MANURES 

have been proposed for the purpose, some of them 
aiming at greater accuracy than others, but they all belong 
to one or other of two types. In one case the separation 
is effected by the movement of a current of water, and in 
the other by the subsidence of the particles through a 
column of water at rest. 

Schone's method is, perhaps, the best of those which 
depend upon the motion of the water. The principle will 
be readily understood from the illustration of the appara- 
tus (Fig. 13). The soil is placed in the elutriator c; a 
current of water passed through it, from below upwards, 
carries off all the particles of a certain size, corresponding 
to the velocity, and is collected in the beaker e. The water 
is supplied from the reservoir a; the velocity of the current 
is regulated by the level of the water in the cylindrical tube 
fc, and is measured by the height to which the water rises in 
the piezometer d. When all the particles in the first group 
have been carried over, the beaker e is replaced by another, 
the speed of the current is increased and a second group, 
consisting of particles of larger size, is obtained. After 
that, a third group, a fourth group and so on are obtained 
in the same way. The size of the particles in each group 
depends upon the velocity of the current, or hydraulic 
value as it is called, and it has been shown that the 
observed and theoretical values correspond very closely. 
Hence, if the speed of the current is accurately known, the 
size of the particles in each group can be calculated and 
does not require to be measured. 

The table on p. 93 shows the hydraulic values of the 
particles of different sizes. 

Osborne's is perhaps the best known method for the 
separation of the particles jby subsidence. Briefly out- 
lined it is as follows : 

A quantity of soil is thoroughly disintegrated and stirred 
with water. When the liquid comes to rest the larger 



THE PHYSICAL PROPEKTIES OF SOILS 



93 



HYDRAULIC VALUE OF PARTICLES OF DIFFERENT SIZES. 



Velocity of Current, 
num. per second. 


Diameter of Particles, 
m.m. 


0-25 


Oil -016 


0-50 


016 -028 


1-0 


028 -040 


2 


040 -050 


4 


050 -077 


8 


077 -122 


16 


122 -166 


32 


166 -306 


64 


306 -500 




particles soon settle to the bottom of the vessel. The 
liquid containing the finer particles in 
suspension is decanted off, allowed to 
settle for a longer period, again decanted 
from the second sediment, and so on. 
The separation, however, is far from 
complete ; the first sediment still con- 
tains particles which properly belong to 
the second; the latter contains some 
particles which properly belong to the 
first and some which belong to the 
third. To adjust these irregularities a 
second series of operations must be 
performed. More water is added to the 
first sediment, stirred, allowed to settle 
for the same time as before, then 
decanted off and added to the second 
sediment. The second sediment is 
stirred with the water decanted from 
the first ; what is deposited after stand- 
ing for a short time is returned to the 
first sediment and what is not deposited FIG. 14. 




94 



SOILS AND MANUEES 



after standing for a farther and longer period is decanted 
off and added to the third sediment. This process 
must be repeated and repeated again until the size 
of the particles in each group is fairly homogeneous. 
The particles in each group are then collected, weighed, 




FIG. 15. 

and their size determined by microscopic measurement. 
The process is easily carried out, but, when many groups 
are required, it is long and tedious. 

When particles of all sizes are uniformly distributed 
through a liquid, the larger particles settle faster than the 
smaller ones, but some of the smaller particles near the 



THE PHYSICAL PEOPEETIES OF SOILS 95 

bottom will be deposited sooner than some of those of larger 
size which have a greater distance to travel. If, however, 
all the particles start from the top of a long column of 
water at the same time, the largest will reach the bottom 
first and the others after longer intervals according to size. 
A method proposed by the author for the separation of par- 
ticles in this way will be understood from the illustration 
(Fig. 14). The apparatus consists simply of a pear-shaped 
flask, about 200 c.c. capacity, w ( i,th a cylindrical neck about 
100 c.m. long. The soil, thoroughly mixed with water, is 
introduced into the flask, and the tube, after being filled up 
to the top with water, is inverted over a pneumatic trough. 
A small porcelain evaporating dish is placed under the 
orifice of the tube so that the sediment may fall directly into 
it. The dish is replaced by others at various intervals of 
time so that groups of particles of different sizes may be 
separately collected in them. 

The method of Bennigsen, of which the above is a modifi- 
cation, is simpler, but it does not provide for the collection 
of the separate groups of particles nor admit of their size 
being mentioned. 

The micrometer used for measuring the size of the par- 
ticles consists simply of a glass disc ruled in squares or, 
more commonly, in simple divisions. It is introduced into 
the eye-piece of the microscope so that the magnified par- 
ticles and the divisions of the scale are seen at the safrne 
time, as in Fig. 15. The size of the particles is then 
easily read off. 



CHAPTER IV 

CHEMISTRY OF SOILS 

CHEMICAL COMPOSITION. 

THE chemical composition of soils is determined by 
chemical analysis, and the subject may be conveniently 
considered from that point of view. Apart from the ques- 
tion of methods, the investigation falls naturally into five 
different sections, vi. : 1. Water and the substances dis- 
solved in it or capable o ( f being extracted from the soil in 
aqueous solution ; 2. Substances soluble in dilute acids, 
commonly regarded as capable o,f being absorbed by plants, 
and often called " available plant foods " ; 3. Substances 
insoluble in dilute acids but soluble in concentrated acids 
these are not immediately available to plants but are 
more or less easily changed into the available state, and 
constitute a store from which the supply of available plant 
foods is replenished ; 4. Organic matter ; 5. The insoluble 
mineral residue, consisting of mere fragments of rock and 
minerals in the fresh condition or only very slightly 
weathered. 

Soil Water. The presence of water in the soil, the con- 
ditions which determine the quantity of it and its influence 
on the physical properties of the soil, have already been 
fully discussed. The proportion of water is estimated by 
exposing the soil to dry air, first at the ordinary tempera- 
ture until it is air dried, and afterwards at a temperature of 
105 C. till it ceases to lose weight. The soil is then said to 
be dry, but it may, and usually does, still contain combined 



CHEMISTEY OF SOILS 



97 



water, chiefly in the form of hydrates. This requires a 
much higher temperature to drive it off. The last traces 
can only be expelled at a red heat. The proportions of 
mixed and combined water found in three samples of soil 
were as follows : 






Sandy 
Soil. 


Loam. 


Clay Soil. 


Loss on air drying ..... 
Additional loss at 105 C 
Combined water, 1 expelled at a red heat . 


Per cent. 

12-5 
1-2 
0-3 


Per cent. 
20-9 
3-2 
2-4 


Per cent. 

26-7 
2-8 
3-6 


Total 


14-0 


26-5 


33-1 



The solvent action of water has also been referred to. 
The majority of substances are soluble in water to some 
extent. The several forms in which some bodies can exist 
often exhibit very different degrees of solubility. 

The dissolving power of water is greatly increased by 
the presence of carbonic acid. A [thousand par.ts of 
pure water can dissolve, at the utmost, only 0*013 parts of 
carbonate of lime. When saturated with carbonic acid it 
can dissolve 0'99 parts, i.e., 76 times as much. 

Soil water is never saturated with carbonic acid but is 
always more or less charged with it, and contains both 
mineral and organic matters in solution. 

There is every reason to believe that these solutions must 
be very dilute, but it is impossible to determine their exact 
concentration or the amount of substances present in a 
soluble state at any particular time. Soluble salts cannot 
be completely washed out of the soil with water. On the 

1 This does not include organic matter, which was estimated 
separately and deducted from the total loss on ignition. 

S.M. H 



98 



SOILS AND MANURES 



11 



u.5 



o 



"^ C^l <N O 

b 4t< t~ 

H IN 



OO^t^OOCOT 1 t- 00 r*< (N t- 
rHi 13^1 l<MrH-(T lTH<MT-l<MT-l 



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rH O O 



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t-Hi l(M(NCOrHCOCOCOCOCO(MT 1 



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^QP.^^T^ l ^ ! i | '^ Hl b T r (r H'iP T - | t~ 

(M rH 



rH rHrHrHrHrHrHrHrHCq 



o 



CHEMISTRY OF SOILS 99 

contrary, most soils can absorb salts from solutions and 
retain them. The drainage water which escapes from the 
soil, however, always contains some salts in solution. The 
amounts of these can be determined and the data are of the 
highest importance as a means of estimating the loss of 
plant foods in this way. The results of analysis, made by 
Dr. Voelcker, of the drainage waters from the experi- 
mental plots of Broadbalk (whea,t) field at Eothamsted, 
are given on page 98. The samples were collected on Decem- 
ber 6th, 1866 ; May 2nd, 1867 ; January 13th, April 21st, 
and December 29th, 1868 ; and the figures given are the 
mean of the five collections. 

Taking the mean quantities of all the constituents, it will 
be seen that ammonia, phosphoric acid and potash are 
present in very small quantity, and that sulphuric acid and 
lime are by far the largest ingredients. It is curious and 
striking that the amounts of soda and lime should be 
respectively so much greater than those of potash and 
magnesia. In general the acid radicles are considerably in 
excess of the bases, but phosphoric acid and lime are con- 
spicuous exceptions to this rule. The quantity of nitric 
acid is less than that of any of the other acids except phos- 
phoric, but it is nevertheless considerable. 

The variation in the Composition of the drainage from 
the several plots may be traced largely to the influence of 
the manures, which were applied as follows : 



Plot 2. Farmyard manure. 

3. Unmanured. 

5. Minerals only. 

6. + ammonia salts (sulphate and chloride). 

7. -f ,, (double quantity). 



8. 
15. 

9. 
16. 



-j- ,, (treble quantity). 

4- ,, (in autumn). 

+ nitrate of soda. 

-j- ,, ,, (double quantity). 



H 2 



100 SOILS AND MANUEES 

Plot 10. Double ammonia salts alone. 
11. ,, ,, + superphosphate. 

,,12. ,, ,, + ,, + sulphate of soda. 

,,13. ,, ,, -f ,, + sulphate of potash. 

,,14. ,, ,, -|- ,, -J- sulphate of magnesia. 

It will be noticed that the drainage from the unmanured 
plot (3) is the most dilute, and that the soluble constituents 
of the manures are all present in appreciable quantities. 
Thus, chlorine, sulphuric acid, nitric acid, and even phos- 
phoric acid, potash, soda and magnesia, are generally found 
in largest quantity in the drainage from the plots to which 
these substances were most largely applied in the manures. 

A large amount of lime always occurs in the drainage, 
even in that from the unmanured plot. The quantity is 
increased by the application of manures of every kind, but 
especially by ammonia salts. The fact is closely connected 
with the general excess of acid radicles over bases other 
than lime. It is evident that the various salts react with 
the lime in the soil, producing soluble calcium salts, e.g., 
the chloride, sulphate and nitrate, which pass into the 
drainage water, thus : 

(NH 4 ) 2 S0 4 + CaC0 3 = CaS0 4 + (NH 4 ) 2 C0 3 
Farmyard manure has a similar effect. The carbonic acid 
evolved by the decomposition of the organic matter con- 
verts calcium carbonate into the more soluble bicarbonate, 
which can be washed out. 

Two highly important consequences arise from these 
changes, viz. : 

1. Loss of lime from the soil ; 

2. Eetention of ammonia and other bases. 

Lawes and Gilbert 1 estimated the loss of lime (calcium 
carbonate) from unmanured land at about 250 Ibs. per 
acre annually. When heavy dressings of manure are 

1 J. E. A. S. E., 1882. 



CHEMISTRY OF SOILS 101 

applied to the land the loss of lime is necessarily much 
greater. It may amount to 1,000 Ibs. per acre. 1 If the 
soil is not naturally well supplied with lime, the reduction 
of the quantity so effected may, in time, seriously impair 
the productiveness. In such cases occasional liming is 
necessary to make good the loss of lime by drainage. 

It has been pointed out that both phosphoric acid and 
potash are found in the drainage water, but the quantities 
of these ingredients are quite inconsiderable, even when the 
land is heavily manured. It is estimated 1 that the mean 
annual loss probably does not exceed 2 Ibs. of the former 
and 10 Ibs. of the latter per acre. The phosphoric acid 
could be replaced in the form of manure for the sum of 
ninepence and the potash for about eighteenpence. The 
total loss of these constituents is not, therefore, a matter of 
serious importance. 

With regard to nitrogen it is quite otherwise. The 
amount of the loss is greater and the substance is much 
more costly. In fact, apart from lime, the loss by drainage 
is practically a question ,of nitrogen. The quantity of 
ammonia is negligible. When separated by the action of 
lime, from the acids with which it is combined in the 
ammonium salts, it rapidly undergoes oxidation and is con- 
verted into nitric acid. The nitrogen of farmyard manure 
and other organic substances ultimately suffers the same 
fate (p. 152). Nitrates are, in effect, the only nitrogenous 
compounds found in the drainage water. According to the 
figures given in the table, the annual loss of nitrogen in the 
form of nitrates from the unmanured plot must be from 
9 to 12 Ibs. per acre, and from the plot which received 
single ammonium salts (containing 43 Ibs. of nitrogen) 20 
to 25 Ibs. per acre, i.e., over 40 per cent, of the amount 
applied. The losses from the plots which received larger 

1 " Book of the Kothamsted Experiments." 



102 



SOILS AND MANUEES 



quantities of nitrogenous manures are of course greater, 
and may amount to from 40 to 50 Ibs. of nitrogen per acre. 
The first-mentioned quantity is about the same as is con- 
tained in the single ammonium salts, or nearly as much as 
is removed from the soil by an average cereal or grass crop. 
Plot 15, to which the ammonium salts are applied in the 
autumn, loses more nitrogen than any of the others. 

It should be kept in mind that all the plots were cropped. 
The losses are therefore much less than from bare soils 
because plants take iip the nitrates and reduce percolation. 
Of course no manure would be applied to bare, i.e., fallow 
land. The amount of nitrates in the drainage water which 
passes through the bare soil of the gauges (p. 67) is deter- 
mined periodically. The average monthly results are as 
follows : 

EAINFALL, DRAINAGE AND NITROGEN, AS NITRIC ACID, IN SAME, 
FROM BARE SOIL 20-INCH GAUGE. 

AVERAGE FOR 26 YEARS, 18781903. 






Rain. 
Inches. 


Percolation. 
Incites. 


Nitric N. 
Parts per 
Million. 


Nitric N. 
Lbs. per acre. 


January . 


2-35 


1-65 


7-5 


2-8 


February . 


1-80 


1-50 


6-8 


2-3 


March 


1-81 


1-00 


69 


1-5 


April 


1-89 


0-50 


7-9 


0-9 


May . 


2-21 


0-60 


8-3 


1-1 


June 


2-40 


0-75 


8-4 


1-4 


July . 


2-55 


0-69 


12-0 


1-9 


August 


2-68 


0-80 


14-0 


2-5 


September 


2-50 


0-85 


16-5 


3-2 


October . 


3-15 


1-80 


14-2 


5-8 


November 


2-55 


2-20 


12-5 


6-2 


December 


2-30 


2-05 


10-2 


4-7 


Total . 


28-19 


14-39 


10-4 


34-2 



Mean. 



CHEMISTRY OF SOILS 103 

It will be seen that the total for the whole year amounts 
to 34 Ibs. of nitrogen per acre. That is about three times 
as much as is lost by the unmanured plot under wheat crop. 

The loss of nitrogen by drainage depends partly upon 
the amount of nitrates in the soil and partly upon the 
amount of percolation. This relation is strikingly illus- 
trated by the curves (Fig. 16). 

During the first four months of the year the actual loss, 
in Ibs. per acre, diminishes almost in the s,ame proportion 
as the amount of percolation ; then it begins to diverge 
because the proportion of nitrates in the soil gradually 
increases whereas the amount of percolation remains prac- 
tically constant till the end of September. The time at 
which the formation of nitric acid takes place is shown 
by the amount, in parts per million, of the drainage water. 
The increase begins in April and continues slowly till 
about the end of June, cT,fter which it proceeds very rapidly, 
reaches a maximum in September, and then declines again 
almost as rapidly (p. 102). The greatest amount of loss 
(Ibs. per acre) takes place in October, November, and 
December. The sharp rise in the first of these months is 
due partly to increased percolation and partly to the much 
larger amount of nitric acid in the soil where it is 
formed rapidly during the months of July, August, and 
September. 

These conclusions are, in the main, confirmed by the ex- 
periments of Way, Deherain and others. 

Absorption of Salts by the Soil. The phenomena of 
absorption, to which reference has been made, are inti- 
mately connected with the question of the fertility of soils. 
It is owing to this power of absorption that only small 
quantities of the soluble substances naturally present in 
the soil, or applied to it as manures, escape in the drainage 
water. Otherwise, the soluble plant foods would soon all 
be washed out of the soil and lost. It has been shown that 



104 



SOILS AND MANUKES 



Nitrates 

Ifbrts per mil/ion) 





Jan. fet Mar. Jpnf May June July /fug. Sept. Oct. Nov. Dec. 

FIG. 16. 



CHEMISTEY OF SOILS 105 

there is no material loss of constituents of manurial value, 
except nitrogen, in this way. 

The main facts have been known since the publication 
of Dr. Way's 1 paper on the subject, and some of them 
long before. For example, it is well known that water 
from deep wells is generally purer than surface water 
from the same locality. Way refers to Bacon's method 
for the elimination of salt from sea water. This was, to 
dig a hole on the seashore above high water mark, deeper 
than the low water mark ; the water which fills the hole 
when the tide comes in is found to be " fresh and potable." 
Way also quotes Huxtable, who had filtered liquid manure 
through ordinary loamy soil and found that it was deprived 
of colour and smell ; "in fact, it went in manure and came 
out water." 

The absorption of salts by the soil can be demonstrated 
by filtering a solution of any potassium salt, of known 
strength, through a long column of soil contained in a 
tube. It will be found that the filtrate is always more 
dilute than the original solution and that the portion re- 
tained by the soil cannot be again completely washed out 
with water. It is, however, only the base that is with- 
drawn from the solution ; the acids, except phosphoric 
and silicic acids, are not retained, but cpme through in the 
filtrate, usually in combination with lime. Even the bases 
are not completely absorbed ; the Amount which comes 
through depends upon the kind of soil and varies with 
concentration of the solution, the time of contact, and, 
possibly, also the temperature. 

The retention of phosphoric acid is probably due simply 
to precipitation in the form of an insoluble salt, but the 
absorption of soluble compounds of potassium, ammonium, 
etc., is not so easily explained. It is found that soils absorb 

1 J E. A. S E., 1850 



106 SOILS AND MANUKES 

more potash from the sulphate than from the chloride, 
and still more from the carbonate. When sulphates or 
chlorides of potassium, sodium, ammonium, etc., are 
applied to the soil, calcium sulphate or chloride appears 
in the drainage water. The salts first react with lime, 
which combines with the acid radicles and liberates the 
bases as carbonates, thus 

K 2 S0 4 + CaC0 3 = CaS0 4 + K 2 C0 3 

The addition of lime to the soil does not, however, 
increase the absorptive power except when the proportion 
of that substance originally present is very small. 

The power of absorption is greatest in soils which con- 
tain clay, humus, and colloidal hydrates of iron and alumina. 
Pure sands have practically no absorptive power at all. 

It is well known that when aluminium hydrate A1(OH) 3 
is precipitated by potash it cannot be entirely freed from 
the reagent by washing with water, and that it is soluble in 
excess owing to the formation of compounds called alumin- 
ates in which the aluminium hydrate acts the part of 
an acid. When carbon dioxide is passed into a solution of 
potassium carbonate, and a solution of aluminate is added 
at the same time, a substance called aluminium-alkali- 
carbonate is precipitated 

A1 2 3 , K 2 + 2 KHCOs = A1 2 3 , K 2 0, 2 C0 2 + 2 KOH 

potassium. alummimn- 

aluminate. alkali-carbonate. 

The retention of potash by aluminium hydrate in the 
soil may be explained in this way, or it may be due to the 
formation of simple aluminates such as KH 2 A10 3 or, pos- 
sibly, compounds of a similar type containing a smaller 
proportion of potash. The aluminates are readily soluble 
in acids, and are decomposed even by carbonic acid. Ferric 
hydrate is also precipitated from salts by potash ; it is not 



CHEMISTEY OF SOILS 107 

soluble in excess of the reagent, but when freshly pre- 
cipitated retains both potash and ammonia. 

The retention of bases by clay, free from colloidal 
hydrates of iron and alumina, is probably due to the forma- 
tion of compounds with hydrated silicates, such as zeolites 
(p. 22) or other secondary products, derived from the 
decomposition, or partial decomposition, of minerals, e.g., 
felspars. 

It has been shown that the natural zeolites possess 
much the same power of absorption as artificially prepared 
hydrated silicates, and that soils in which silicates soluble 
in acids are most abundant have the highest absorptive 
power. The absorptive power of clay is, therefore, gener- 
ally attributed to the presence of these substances. 

If the absorption of potash by aluminium hydrate be 
due to the formation of aluminates, it is to be expected that 
kaolin, which is a partial hydrate of alumina, 

f SiO OA1 (OH) 2 
1 SiO OA1 (OH) 2 

and other similar compounds would exhibit perhaps in 
modified degree a similar power of absorption. This view 
is consistent with the fact that the removal of soluble sili- 
cates from the soil by treatment with hydrochloric acid 
does not destroy the power of absorbing alkaline hydrates 
and carbonates, but, as it causes the simultaneous removal 
of the lime, it necessarily destroys the power of absorbing 
neutral salts. 

Humus also exhibits considerable power of absorbing 
both potash and ammonia, but, as the humates of these 
bases are soluble in water, it is probable that the 
phenomena are due to purely physical and not to chemical 
causes. Whitney has called attention to the importance 
of this aspect of the question. He points out that clays, 
calcareous soils and humus present an enormous extent 



108 SOILS AND MANURES 

of internal surface (p. 55), and that the power of attrac- 
tion must be very great. It is probable, however, that 
such purely mechanical retention would affect the salts 
as a whole, :and one .salt nearly as much as another./ 
Mechanical causes, though they doubtless contribute to 
the retention of salts in the soil, must, therefore, be 
regarded as of minor importance. 

Organic Matter. The organic matter of the soil is 
mainly of vegetable origin and is derived from the remains 
of previous generations of plants. In the case of soils 
under cultivation the crops are removed, but, as a rule, 
a portion of the plants sometimes the roots, sometimes 
the leaves is left in or on the land. The removal of the 
crops is at least partially compensated by ploughing under 
weeds and green manures, and by the application of farm- 
yard manure. The organic matter rapidly decays owing 
to the action of fungi, bacteria and ferments of various 
kinds ; the organised structure is destroyed and it becomes 
intimately mixed with the other particles of soil. When 
organic matter accumulates under water the vegetable 
structure is, to a large extent, preserved and peats are 
formed. The disappearance of the organised structure in 
the open soil has been attributed to the action of worms and 
larvae, but it seems more probable that it is due simply to 
more rapid oxidation, and its preservation under water 
to the exclusion or limiting of the air supply. The decom- 
position of organic matter certainly proceeds most rapidly 
on soils that are most exposed to oxidation open sandy 
soils and the presence of clay favours its accumulation. 
After passing through various intermediate stages, the 
organic matter finally becomes completely oxidised, the 
oxides of carbon and hydrogen are volatilised, and the 
nitrogen and ash constituents become available to growing 
plants. 

Humus is not a definite compound but a complex mix- 



CHEMISTEY OF SOILS 



109 



ture of products marking a transition stage in the gradual 
decay of organic matter. Owing, however, to the fact that 
its origin is practically always the same, it exhibits more 
or less specific properties, and the variation in composi- 
tion does not exceed definable limits. Its more important 
physical properties are dark colour, low density, great 
porosity, and high specific heat. It therefore affects the 
properties of soils in the manner already described. 

It gives rise to colloidal bodies of an acid nature, often 
referred to collectively as humic acid or geic acid. It is 
generally accepted, however, that this is not one but several 
distinct substances. Names and formulae have been 
ascribed to those which, it is claimed, have been isolated 
and examined, but the opinions of various authorities on 
the subject are widely divergent, and their results are so 
inconsistent that it is difficult to repose confidence in any 
of them. According to Mulder, humus is mainly composed 
of six different compounds, known respectively as humin, 
humic acid, ulmin, ulmic acid, crenic acid, and apocrenic 
acid. Stockbridge gives the same formula for humin and 
humic acid, and also for ulmin and ulmic acid. Another 
authority declares that humic acid and ulmic acid are one 
and the same substance, and proposes a formula which 
does not correspond with those of the others for either sub- 
stance. The formulae proposed respectively by Mulder and 
Stockbridge for the various substances are as follows : 






Mulder. 


Stockbridge. 


Humin . 


C40 HSO Ola 


C 21 H 24 Oi 2 + 3 H a O 


Humic acid . 


40 H24 Oi2 


Same 


Ulmin . 


C4Q H;j2 Ol4 


^40 H28 Ol2 -j- H<jO 


Ulmic acid . 


040 Hag Oi2 


Same 


Crenic acid . 


C 24 H 24 Ou, + 3 HaO 


Cl2 Hi2 O 8 


Apocrenic acid .* 


C24 Hj2 Oi2 ~\~ -hl^O 


C>24 H24 Ol2 



110 SOILS AND MANUEES 

The various substances are all closely related ; they 
undergo constant change, and probably one is derived 
from another by processes of oxidation and dehydration. 
They are obtained as a whole by extracting hurnous soil 
with ammonia solution and reprecipitating with acid. The 
product, commonly called " humic acid," is an amorphous 
colloidal substance, slightly soluble in water, has an acid 
reaction and decomposes carbonates. It unites readily with 
ammonia and alkalis forming soluble compounds called 
"humates." The compounds with lime and other bases 
are insoluble, and in the preparation of humic acid the 
bases should be removed by first extracting the soil with 
hydrochloric acid. In preparing calcium hurnate Schloe- 
sing neutralised the dark coloured ammoniacal extract 
till a permanent precipitate of humic acid just began to 
form, and when this was redissolved, he added calcium 
chloride solution and obtained a precipitate of calcium 
humate. It is a colloidal body of great cementing power 
(see p. 87). 

Apart from its influence on the physical properties of the 
soil, humus owes its great importance in agriculture chiefly 
to the nitrogen it contains. The nitrogen is not included 
in any of the formulae ascribed to the supposed consti- 
tuents of humus, because it has been assumed that it exists 
in the form of a basic radicle in combination with the acid 
compounds. However this may be, it appears to be pre- 
sent in an insoluble form and is not, therefore, liable to 
loss by drainage, but it is rapidly converted into soluble 
compounds and becomes available to plants as the humus 
decays. The proportion of nitrogen, like that of the other 
constituents of humus, is variable. Mulder found from 
2*5 to 4'0 per cent., and Kostytcheff from 4*0 to 6*55 per 
cent, in different samples. Owing to the elimination of 
carbon and hydrogen by oxidation there is a tendency for 
the proportion of nitrogen to increase as the humus decays 



CHEMISTEY OF, SOILS 111 

but the soluble compounds do not accumulate in soils : 
they are lost in the drainage water if not absorbed by 
plants. 

The organic matter of soils is oxidised and' driven off 
on heating, and the loss on ignition is often put down as 
organic matter. No just estimate of the amount can, how- 
ever, be formed in this way because combined water and, 
in some cases, carbon dioxide from carbonates may be 
volatilised at the same time. For example, a soil which 
lost 5 '8 per cent, on incineration, after drying at 100 C., 
was found to contain less than 1 per cen,t. of true 
organic matter. If the organic matter were of constant 
composition the quantity of it present in any instance 
could be easily deduced from the proportions of organic 
carbon or hydrogen. Unfortunately this is not the case, 
but calculations based on the assumption 1 that it contains 
58 per cent, of the former and 5 per cent, of the latter 
have given results in accordance with known facts, and, 
in most cases, are probably not far from the truth. The 
proportion of humus bears no constant relation to that 
of the total organic matter and, as it has a much higher 
agricultural value than fresh vegetable matter, it should 
be estimated separately. This can be done by Grandeau's 
method, or some modification of the same, i.e., by weighing 
the dry residue left after evaporation of the ammoniacal 
extract. 

Lime. The calcareous matter or " lime " of soils con- 
sists of calcium carbonate. It should not be confused with 
other calcium salts also found in soils. The latter have 
entirely different properties and produce different effects. 

The proportion of lime in soils varies very widely. In 
some it forms the predominant constituent, while others 

1 The composition of humus differs from that of the total organic 
matter. About 45 per cent, of carbon and 4'5 per cent, of hydrogen 
are probably nearer the average for the former substance. 



112 SOILS AND MANUBES 

contain practically none at all. Average fertile soils may 
contain from 2 to 10 per cent, of lime. Those which 
contain less than 1 per cent, require careful management, 
especially if they contain much clay or organic matter. In 
sandy soils lime is not so necessary for fertility, and it is 
usually present in smaller quantity in soils of that class. 

The calcareous ,matter of soils is derived from shells, 
chalk, limestone, marls, and the lime artificially applied 
in case of need. Some limestones are hard and crystalline, 
but chalk is amorphous, soft, friable and very porous. 
Chalky soils are usually very poor, but, owing to their great 
porosity, retain moisture and yield a short sweet herbage 
well adapted for pasturing sheep. The downs are almost 
entirely devoted to this kind of cultivation. 

Calcium carbonate is practically insoluble in pure water, 
but combines with carbonic acid to form a more soluble 
bicarbonate, 

CaCOs + C0 2 + H 2 = CaH 2 (C0 3 ) 2 

That is why it dissolves in water charged with carbonic 
acid. The bicarbonate, however, is quickly decomposed 
on boiling the solution and more slowly at ordinary tem- 
peratures ; the carbonic acid is evolved, and the normal 
carbonate is reprecipitated, 

CaH 2 (C0 3 ) 2 = CaC0 3 + H 2 + C0 2 

The stalactites seen in the caves at Cheddar and other 
limestone districts, and in grotto structures, have been 
formed in this way, and the formation of limestones from 
deposits of shells is attributed partly to the same cause. 
The precipitation of calcium carbonate consequent upon 
the evaporation of carbonic acid or its absorption by other 
substances from bicarbonate solutions produces a cement- 
ing effect on sandy soils. Carbonate of lime sometimes 
becomes infiltrated in this way through a mass of loose sand 



CHEMISTRY OF SOILS 113 

in quantity sufficient to form a cementing matrix, and the 
mass becomes consolidated into a hard sandstone. It is 
in this way also that the calcareous pans are formed in 
subsoils. 

Carbonic acid is, the feeblest of all acids, in the sense that 
it is displaced from its compounds with bases by every other 
acid. Calcium carbonate is no exception t!o this rule ; it 
reacts with acids forming the corresponding calcium salts, 
and carbonic acid is given off. 

CaC0 3 + 2 HN0 3 = Ca(N0 3 ) 2 + H 2 + C0 2 

Calcium Nitric acid. Calcium nitrate. Carbonic 

carbonate. acid. 

It is to this property and its insoluble, neutral character 
that lime owes its greatest value as a constituent of the 
soil. It is important that a neutral condition should be 
maintained in the soil. Both acids and alkalis interfere 
with the growth of plants and with the activity of lower 
organisms concerned with fertility. Acid substances are 
constantly Jbeing formed by the decay of organic matter, 
and sometimes also as a result of chemical changes in 
certain minerals, and, if allowed to accumulate, produce 
that " sour " condition which is inimical to fertility. The 
acids cannot be neutralised by addition of alkalis, for the 
excess of alkali would produce a disastrous effect on the 
physical condition of clay, and is probably as deleterious 
to the plants as acids. In short, the cure would be worse 
than the disease. 

Calcium carbonate, however, for the reasons given above, 
serves the purpose equally well. The acids unite with 
the lime, forming neutral compounds, and the carbonic 
acid is expelled. Being itself an insoluble substance, of 
neutral reaction, a large excess of it can be stored in the 
soil without detriment to the growing plants or lower 
organisms, and, as a rule, with great benefit to the physical 

S.M. x 



114 SOILS AND MANUEES 

properties of the soil. In fact, the calcium carbonate, 
though it is a neutral salt, can act as a potential base and, 
if present in sufficient quantity, will maintain the neutral 
condition of the soil. 

An acid or alkaline reaction of the soil is easily detected 
by pressing a piece of litmus paper lightly against the 
fresh moist earth, but very often a slight acidity cannot 
be detected after the soil has been air dried. 

Alkaline soils are of comparatively rare occurrence and 
are practically unknown in this country, but in certain 
parts of the Western States of America,where, owing to the 
deficient rainfall, the soils are never thoroughly saturated, 
and there is little or no drainage, alkaline substances 
chiefly sodium carbonate accumulate and impart their 
characteristic reaction to the soil. Many of these soils, 
which would otherwise be capable of great fertility, are 
rendered almost barren by the deleterious effects of the 
alkali. The only complete remedy is to wash out the 
alkaline salts by irrigation, but their effects can be miti- 
gated by the addition of gypsum. This substance reacts 
with sodium carbonate, forming neutral sodium sulphate 
and calcium carbonate, as shown by the equation 

Na 2 C0 3 + CaS0 4 = Na 2 S0 4 + CaC0 3 

Sodium Calcium Sodium Calcium 

carbonate. sulphate. sulphate. carbonate. 

Except in the case of very open sandy soils, in which 
organic matter is so rapidly oxidised that very little humus 
is ever formed, there is a general tendency for the soils in 
this country to become " sour " unless the acid substances, 
formed by the decay of organic matter, are neutral- 
ised by lime. If the soil does not naturally contain enough 
calcium carbonate to effect this, frequent liming becomes 
a paramount necessity. The ruddy brown deposit of ferric 
hydrate commonly seen in the drainage waters from moor- 



CHEMISTEY OF SOILS 115 

land soils, which contain much organic matter and little 
lime, is produced by the oxidation of iron dissolved out 
by acids from ferruginous minerals in the soil. By .a 
similar process ferric hydrate sometimes accumulates in 
the subsoil of cultivated lands. It cements the particles 
together and, as it gradually becomes dehydrated, forms 
an impervious indurated layer called "iron pan" or 
" moor-band pan." Pan formation may also be due to the 
presence of compounds of manganese, but these are of 
rarer occurrence. 

The degree of acidity can be determined by mixing a 
quantity of soil with alkali solution of known strength, 
and then estimating the amount of alkali neutralised by 
the soil. In soils of neutral reaction the capacity for 
neutralising acids can be estimated in a similar manner. 
The soil is mixed with acid of known strength, and 
*the amount of acid neutralised is then determined in 
the usual way. This value when calculated as calcium 
carbonate is often called "available lime," i.e., lime 
available to neutralise acids. The experiment, however, 
should be performed on a fresh sample, because it is found 
that a soil which gives an acid reaction while moist will, 
in some cases, after air drying, absorb a certain quantity 
of acid. This makes it appear as if the soil contained some 
available lime when, as a matter of fact, it does not. In 
any case the result is only approximate and is affected 
by the strength of the acid employed. 

The presence of lime in soils promotes oxidation. It 
causes the organic matter to decay faster and favours the 
production of nitric acid, with which it unites, forming 
calcium nitrate. In the absence of a sufficient amount of 
lime, the activity of the nitrifying organisms is reduced, 
putrefactive decomposition takes place instead of oxida- 
tion, and sulphides and other substances poisonous to 
vegetation result. The question of nitrification will be 

i 2 



116 SOILS AND MANURES 

more fully considered later on. The oxidation of com- 
pounds of iron and other minerals also takes place more 
rapidly when the soil is well supplied with lime. 

Not only does lime unite with free acids, but it also reacts 
with various salts in the soil, combining with the acid 
radicles and liberating the bases. This appears to be a 
condition precedent to the absorption of phosphoric acid 
and of bases by the soil. If it does not occur, various salts 
used as manures become comparatively ineffective, or even 
positively injurious. The calcium salts so formed are 
mostly soluble in water and pass into the drainage, and 
considerable loss of lime results. 

In all these ways lime adds greatly to the effectiveness 
of manures. In cases of reduced fertility the soils are 
sometimes singularly unresponsive to all kinds of manuria] 
treatment, and, though the addition of lime itself may pro- 
duce but little improvement, it often makes the soil respond 
generously to the manures subsequently applied. Farmers 
are sometimes deceived in this way. In order to test 
whether or not a soil requires lime, a small dressing is 
applied to a corner of a field, and if it produces no visible 
result they not unnaturally conclude that lime is not 
required. 

Calcium is an essential constituent of plants, and the 
fact that the lime in the soil is, therefore, a possible source 
of plant food has been, perhaps, unduly emphasised. 
Plants may obtain this element from lime, but it is pro- 
bable that, if that ingredient were absent altogether, they 
could obtain all the calcium they require from calcium 
sulphate and other soluble salts. In fact, there is some 
reason to believe that most of the calcium found in plants 
is derived from that source and not directly from the car- 
bonate. The great improvement in the quality of the 
herbage, which often follows an application of lime to 
pastures, is probably due mainly to its effects on the soil, 



CHEMISTRY OF SOILS 117 

and only to a slight extent, if at all, to the increase in the 
quantity of calcium compounds. 

Some of the less desirable plants sedges, rushes, etc. 
are better adapted than grasses for growth in sour soils. 
When the adverse conditions are removed by application 
of lime, the growth of grasses is encouraged and the others 
are crowded ,out. A dressing of lime will often make a 
pasture white with clover though not a blade of it was to 
be seen for years previously. The large proportion of 
calcium found in clovers has given rise to a widely spread 
idea that this effect is due to the increase of calcium com- 
pounds in the soil. The same effect has,, however, been 
observed on soils known to contain a considerable propor- 
tion of calcium salts other than carbonate. It is not pro- 
duced by an application of gypsum 1 or similar compounds 
but only by lime. The favourable influence of lime upon 
the growth of clovers must, therefore, be attributed mainly 
to its action on the soil rather than to a'ny direct effect upon 
the plant. It is probable that the proportion of soluble 
calcium salts in the soil is ample, in all but the rarest cases, 
to provide for the requirements of the crops so far as the 
supply of this element of plant food is concerned. Never- 
theless, the soils may be very deficient in lime. 

A dressing of lime is frequently applied to pastures for 
the destruction of moss, and in some cases is found to be 
very effective. Moss is found chiefly in low lying or badly 
drained,wet land, and it grows all the better when it has not 
to compete with other plants for the available space and 
food. There is no reason to suppose that the presence of 
lime in the soil is actually inimical to the growth of moss. 
It flourishes on land that is not conspicuously deficient 
in lime, and applications of that substance often make very 

1 Gypsum is said to have a favourable effect on the growth of 
clovers in some cases, but such is not the writer's experience. 



118 SOILS AND MANUEES 

little apparent difference to it. There is, however, good 
reason to believe that moss is more indifferent to the 
absence of lime than grasses and other cultivated plants. 
A sour condition of the soil therefore favours the growth of 
moss by discouraging that of the other plants. Under these 
circumstances lime will have a beneficial effect. It neutral- 
ises the acids in the soil and so renders the conditions 
more favourable for the growth of grasses and clovers, in 
this case also the improvement is due to the action of the 
lime upon the soil rather than to any direct influence upon 
the plants, and it cannot be expected to take place unless 
the soil is deficient in lime. 

In order that lime may be thoroughly incorporated with 
the soil it must be reduced to a very fine state of division. 
For this reason it is generally applied to the land in the 
form of dry slaked lime. The limestone is " burned," i.e., 
it is heated to a very high temperature, and so decomposed 
into calcium oxide, or quicklime, and carbon dioxide. The 
latter substance is a gas, and is driven off, and the quick- 
lime remains in lumps of the same size and shape as the 
pieces of limestone before burning, but considerably 
lighter. The change may be represented by the following 
equation : 

CaC0 3 CaO + C0 2 

Carbonate of lime Calcium oxide Carbon dioxide, 
(limestone). (quicklime). 

The quicklime is carried out to the land, deposited in 
heaps, and allowed to slake, i.e., to become converted into 
the hydrate by absorbing moisture from the air. This 
causes it to fall into a fine dry powder which is easily spread 
over the land and mixed with the soil. The chemical 
change can be represented by an equation, thus 

CaO + H 2 = Ca(OH) 2 

Quicklime. Water. Slaked lime (calcium hydrate). 



CHEMISTEY OP SOILS 119 

But a further change soon takes place; the calcium 
hydrate, or slaked lime, combines with carbon dioxide in 
the air and re-forms calcium carbonate, thus 

ga(OH) 2 + C0 2 CaC0 3 + H 2 

Slaked lime. Carbon dioxide. Calcium carbonate. Water. 

The net result of all these changes is, therefore, to repro- 
duce the same substance with which they began. All that 
is accomplished is to obtain the substance, calcium car- 
bonate, in fine state of division suitable for admixture with 
the soil. 

Within the last few years a substance called ground lime 
has been put on the market and has been somewhat exten- 
sively employed. This is simply quicklime mechanically 
ground to a fine powder. It is spread on the land in that 
form and there becomes first slaked and then carbonated 
as above described. It is said to be more effective than the 
ordinary lime, and consequently a smaller quantity wil] 
suffice for a given purpose. In cases where difficulties of 
transport are encountered this may be an important con- 
sideration, but it is more expensive to purchase and 
requires very careful handling. If allowed to become damp 
the lime slakes, the bags burst, and damage may be done. 
When first applied it probably acts more vigorously on the 
organic matter in the soil, but this effect must be transient, 
and its chief advantage over ordinary lime is probably 
due to the more thorough distribution of the lime through 
the soil when it is applied in this form. 

Lime is used in gasworks to absorb the sulphuretted 
hydrogen and carbon dioxide from the gas produced by 
destructive distillation of coal. The spent product, called 
" gas lime,'.' is sometimes used for agricultural purposes. 
It contains from 10 to 20 per cent, of calcium hydrate, from 
20 to 25 per cent, of carbonate, varying quantities of 
sulphide, sulphite, thiosulphate, and other impurities. The 



120 SOILS AND MANUKES 

sulphur compounds have a deleterious effect on vegetation, 
but on exposure to air they are all oxidised ultimately to 
sulphates, which are harmless. Gas lime should not, there- 
fore, be used in the fresh state, but should be composted 
for some time before it is applied to the land. As a source 
of lime it cannot be considered suitable for general use. 

In the ordinary course of analysis, the next step, after 
removal of the organic matter, is to extract the substances 
soluble in concentrated acids. The reagent most commonly 
employed for this purpose is hydrochloric acid of moderate 
strength. For most of the substances it is a better solvent 
than any other, but, for several reasons, extraction with 
nitric acid is preferred for estimation of phosphoric acid 
and some other constituents. The process of digestion 
occupies several days ; when it is complete, the extract 
containing various substances in solution is filtered off and 
the insoluble residue is collected and weighed. 

The insoluble mineral residue. The residue which 
remains after treatment with hydrochloric acid, often 
called sand or simply insoluble matter, consists of frag- 
ments of undecomposed rock and minerals and substances 
which, like quartz, are permanently insoluble. It com- 
prises a very large proportion usually over 80 per cent., 
and not infrequently over 90 per cent. of the total dry 
matter. In soils largely composed of calcareous matter, 
or vegetable matter, the insoluble mineral residue is, of 
course, much less. 

As a constituent of the soil its influence is purely 
mechanical, i.e., it only affects the physical properties. 
It must be remembered, however, that it is the source from 
which the substances found in the extract, presently to be 
described, are derived and constantly, if gradually, replen- 
ished. It is, in fact, the ultimate source from which all 
the mineral elements of plant food in the soil originate. 
Its composition generally resembles that of the parent 



CHEMISTEY OF SOILS 121 

rocks, but is only of importance when the ultimate pro- 
perties of the soil are under consideration. 

Two methods are chiefly employed in the investigation 
of the insoluble matter. In one, the silica is volatilised 
in the form of silicon fluoride by treatment with hydro- 
fluoric acid, and the bases which remain behind are dis- 
solved in acid and examined in the usual way. In the other 
method, the silica is converted into a soluble silicate by 
fusion with podium carbonate. This compound is then 
decomposed with acids ; the silica is precipitated and the 
bases are dissolved and can be examined as before. 

The Total Acid Extract. In analyses of soils the total 
amounts of all the mineral matters which can be dissolved 
by concentrated acids are generally classed together and 
may be conveniently referred to as the total acid extract, 
without reference to the particular kind of acid that may 
have been employed to dissolve any one or more of them. 
The results of analyses of the fine earth of certain typica] 
soils, showing the proportion of organic matter, insoluble 
mineral residue and total acid extract, and the composition 
of the last, are given in the tables on p. 122. 

It will be seen that the proportion of total acid extract 
varies considerably in different classes of soil. In general, 
it is greatest in soils which contain much lime and least in 
sandy soils. In ordinary non-limey soils from 5 to 15 per 
cent, of 4)he dry fine earth is usually soluble in concen- 
trated acids. Oxides of iron and alumina are the principal 
ingredients, lime and, sometimes, magnesia are present 
in considerable quantity. The proportions of the other 
ingredients are relatively small usually less than 1 per 
cent., and not infrequently less than ^ per cent. The 
large proportion of potash found in the stiff clay soil is 
exceptional. One of the humous soils contains much 
less potash than ;the other. Soils of that class are not 
infrequently deficient in potash. The largest proportion 




122 



SOILS AND MANUEES 



ANALYSIS OF SOILS (DRY FINE EARTH). 






Sandy 
-Soil. 


Sandy 
Loam. 


Medium 
Loam. 


Stiff Loam. 


Heavy 
Clay. 




Per cent. 


Per cent. 


Per cent. 


Per cent. 


Per cent. 


Potash . 


17 


32 


53 


45 


1-58 


Soda . 


03 


26 


74 


39 


97 


Magnesia 


09 


26 


1-24 


65 


01 


Lime 


12 


45 


2-41 


95 


05 


Oxides of iron 


3-24 


2-75 


4-72 


3-47 


6-42 


Alumina 


2-56 


2-91 


3-06 


4-12 


8-72 


Phosphoric acid 


08 


0-6 


21 


09 


11 


Carbonic acid 


| 


1-28 





13 


Sulphuric acid 








09 


10 


06 


Chlorine 





13 


03 


02 


96 


Insoluble matter . 


91-02 


86-35 


77-63 


83-44 


72-15 


Organic matter 


2-21 


5-14 


6-65 


5-88 


8-78 






Marly Soil. 


Chalky 
Soil. 


Calcareous 
Soil. 


Humous 
Soil. 


Humous 
Soil. 




Per cent. 


Per cent. 


Per cent. 


Per cent. 


Per cent. 


Potash . 


36 


32 


97 


01 


16 


Soda 


35 


28 


16 





14 


Magnesia 


52 


20 





20 


67 


Lime 


11-15 


24-32 


30-55 


1-01 


1-40 


Oxides of iron 


5-96 


1-24 


3-32 


6-30 


5-67 


Alumina 


5-96 


1-35 


6-00 


9-30 


3-29 


Phosphoric acid 


0-38 


12 


01 


13 


31 


Carbonic acid 


8-77 


10-37 


23-91 








Sulphuric acid 


04 


22 


01 


17 


19 


Chlorine 


76 














Insoluble matter . 


55-52 


58-34 


28-77 


72-80 


64-71 


Organic matter 


10-50 


3-33 


6-33 


10-08 


23-03 



of phosphoric acid in any of the examples quoted is 0'38 
per cent. ; the calcareous soil contains only O'Ol per cent. 
It is obvious, therefore, that some of the most important 
ingredients of the soil are normally present in very small 
proportion. It must be remembered, however, that a small 
percentage corresponds to a large quantity of the substance 
in the total mass of soil, and that the quantities required 



CHEMISTEY OP SOILS .123 

by plants are relatively minute. Thus, a crop of mangels, 
which requires more phosphoric acid and potash than any 
other, removes from an acre of land only about 50 Ibs. of 
the former and 300 Ibs. of the latter. If the total mass 
of an acre of soil 9 inches deep be taken as 2,500,000 
Ibs. which is a moderate estimate 0'002 per cent, of 
phosphoric acid and 0'012 per cent, of potash would be 
sufficient to provide these quantities. It is, of course, 
essential that the soil should contain a considerable excess 
of the various substances over and above what is actually 
required by the crops, and the point may be, perhaps, better 
illustrated by putting the case in another way. In a mass 
of 2,500,000 Ibs. O'l per cent, is equal to 2,500 Ibs., which 
is more than eight times the amount of potash and fifty 
times the amount of phosphoric acid required by a mangel 
crop. The ordinary grain crops require only about h,alf 
the quantity of phosphoric acid and about a tenth part of 
the potash that mangels require. 

Available Plant Foods. The total acid extract is, how- 
ever, a matter of secondary interest. A large proportion 
of the phosphoric acid and other ingredients dissolved by 
concentrated acids is present in the soil in a non-available 
state, i.e., in such a state of combination that it cannot 
be assimilated by plants. The total acid extract does not, 
therefore, afford a reliable indication of the capacity of 
soils to provide for the requirements of the. crops. For 
example, it has been found that soils which, experience 
shows, stand in need of potash manures often contain as 
much potash, soluble in concentrated acids, as soils which 
do not. Exactly what this so-called " .available state " may 
be is not known. That it is closely connected with the solu- 
bility of the compounds is obvious, and solubility depends 
partly on physical and partly on chemical conditions. For 
example, there is a great difference between the solu- 
bility of crystalline apatite and that of freshly precipitated 



124 SOILS AND MANURES 

phosphate of lime. Calcium phosphate, again, is probably 
more readily soluble than phosphates of iron and alumina, 
especially if the latter have been dried or partially dried. 
All that can be said with certainty is that some substances 
which are not soluble in water can be assimilated by 
plants, and that much of what is dissolved by concentrated 
acids cannot. In any case, the available state cannot be 
defined in terms of solubility, because plants differ in their 
assimilative capacities; what is available to one is not 
available to another. There is, therefore, no absolute 
available state at all. Attempts to define it in general 
terms are apt to be misleading, and methods for estimating 
the amount of " .available plant food," in the strict sense, 
are impossible. 

This, however, does not preclude the possibility of deter- 
mining by chemical analysis whether or not a soil stands 
in need of potash or phosphatic manures. It has been 
established that .this cannot be predicted with certainty 
from the amounts of potash and phosphoric acid extracted 
from the soil either by strong acids or by pure water, and 
the fact suggests that dilute acid might prove a more suit- 
able solvent. Acids of different kinds and various strengths 
have been tried by several authorities, but that which has 
been most generally approved in recent years is a cold, 
1 per cent, solution of citric acid. This solvent was 
proposed by Dyer after a careful examination of the acidity 
of the root sap of a number of different species of plants. 
It was thought that it would nearly imitate the dissolving 
action of the roots, and the phosphoric acid and potash 
extracted by it have been called " available plant foods." 
The term is, perhaps, an unfortunate one, but, as it is 
in common use, it must be accepted. It must be clearly 
understood, however, that no sharp line of distinction can 
be drawn between what is available and what is not. Dyer 
applied the reagent to soils of known character and 



CHEMISTRY OF SOILS 125 

recorded history, and showed that it affords a much clearer 
indication of the manurial requirements of the soil than 
can be obtained from the total acid extract. This is well 
illustrated by comparison of the results obtained by the 
two methods when applied to samples of soil from the 
plots at Bothamsted, which differed only in the manurial 
treatment to which they had been subjected. The plot, 
which has been continuously unmanured for thirty-eight 
years, yields smaller crops than those to which phosphatic 
and potash manures have been regularly applied for the 
same length of time, owing to the partial exhaustion of 
these constituents in the first. It was to be expected, 
therefore, that smaller proportions of phosphoric acid 
and potash would be found in the unmanured plot than 
in those to which the manures had been applied. As a 
matter of fact this proved to be the case, and a certain 
difference was revealed even in the total acid extract, but 
the difference in the proportions extracted by the dilute 
acid is much greater and, therefore, more easily distin- 
guished. The results obtained are shown in the tables 
(p. 126). 

It will be seen that while the amounts of potash and 
phosphoric acid extracted by dilute citric acid are in all 
cases much less than those extracted by strong hydro- 
chloric acid, they are, nevertheless, a much better guide 
to the manurial requirements of the soil. This becomes 
more apparent when the ratios of the percentages are con- 
sidered. Thus, the dilute acid extracted from the plot 
treated with phosphatic manure nearly eight times as much 
phosphoric acid as from the unmanured plot, whereas 
strong hydrochloric acid extracted less than twice as much. 
The dilute acid also extracted ten times as much potash 
from the plot treated with potash manures as it did from 
the unmanured plot, whereas strong hydrochloric acid ex- 
tracted only 1*7 times as much. It will be noticed that 



126 



SOILS AND MANUKES 



PERCENTAGE OF PHOSPHORIC ACID AND POTASH EXTRACTED BY STRONG 
HYDROCHLORIC ACID AND BY A 1 PER CENT. SOLUTION OF CITRIC 
ACID FROM THE SOIL OF KOTHAMSTED PLOTS. 





Phosphoric Acid 
Extracted by 


Potash. 


Continuously manured 








Extracted by 


for 38 yeors. 


Strong 


Dilute 








HCl. 


Citric Acid. 


Total. 


Strong 


Dilute 










HC1. 


Citric Acid. 




Per cent. 


Per cent. 


Per cent. 


Per cent. 


Per cent. 


No manure 


099 


0055 


1-448 


183 


0036 


Phosphatic manure 


128 


0436 


1-500 


204 


0065 


Potash manure 


121 


0100 


1-695 


318 


0366 


Phosphatic and 












potash manures . 


189 


0538 


1-718 


300 


0340 



SAME IN POUNDS PER ACRE. 





Lbs. 


Lbs. 


Lbs. 


Lbs. 


Lbs. 


No manure . . 


2503 


139 


36,604 


4626 


91 


Phosphatic manure 


4601 


1170 


37,918 


5157 


165 


Potash manure 


3059 


253 


42,84b 


8039 


925 


Phosphatic and 












potash manures . 


4778 


1360 


43,429 


7584 


859 



SAME, SHOWING THE KATIO OF THE PERCENTAGES (THE AMOUNT IN 
THE UNMANURED SOIL is TAKEN AS 1 IN EACH CASE). 



Unmanured . 


1-00 


1-00 


1-00 


1-00 


1-00 


Phosphatic manure 


1-82 


7-92 


1-03 


1-11 


1-83 


Potash manure 


1-21 


1-82 


1-17 


1-73 


10-16 


Phosphatic and 












potash manures . 


1-89 


9-78 


1-19 


1-64 


9-44 



the total potash, i.e., inclusive of that insoluble in strong 
acids, conveys no idea at all of difference in the capacities 
of the soils, for the difference in the amounts in the several 
plots is barely perceptible. Dyer concluded that, as a 
general rule, soils which contain less than O'Ol per cent. 



CHEMISTRY OF SOILS 127 

of phosphoric acid and 0'005 per cent, of potash, soluble 
in the dilute citric acid reagent, stand in need of phos- 
phatic and potash manures. 

The method has attracted the attention of agricultural 
chemists in all parts of the world, and has been very exten- 
sively employed in this country. It has been applied to 
many different types of soil, and has been compared with 
other methods. In general, the result has been to confirm 
Dyer's views; the 1 per cent, solution of citric acid 
gives more trustworthy information regarding the manurial 
requirements of soils than any other solvent yet examined, 
but probably the standards proposed are not generally 
applicable to all soils alike. 

CHEMICAL CHANGES. 

Chemical changes of one kind or another are constantly 
taking place in the soil, and sooner or later most of the 
constituents are affected. Many undergo slow but con- 
tinuous modification as a result of the operation of the 
various forces mechanical, chemical and biological to 
which ;they are exposed. Soluble salts are produced by the 
decomposition of insoluble minerals ; compounds are 
oxidised, reduced and dehydrated ; pans are formed and 
disintegrated ; humus accumulates and disappears, and 
various constituents of the soil react upon each other and 
upon substances applied to it. 

A few of the changes are simple, direct and easily under- 
stood, but some are reversible, and most of them are very 
complex and involved in obscurity. Those which take place 
slowly are all the more difficult to follow by reason of the 
numerous intermediate stages which can sometimes be 
recognised but cannot be determined. Final products can 
be traced to original compounds and the character and 
direction of the change made clear, but the number of 



128 SOILS AND MANURES 

definite intermediate compounds and the composition of 
the same are often entirely unknown. 

Decomposition of Minerals. Among the most important 
of the chemical changes which take place in the soil are 
those by which the constituents of insoluble minerals are 
rendered available to plants. Carbonic acid and water 
take part in the reactions which are also greatly accelerated 
by the processes of mechanical pulverisation previously 
described. 

The formation of kaolin from felspars may be taken as 
typical of the changes which complex silicates undergo. 
In the case of anorthite, the lime felspar, it may be repre- 
sented by the following equation 

Ca A1 2 Si 2 8 + 2 C0 2 + 3 H 2 = Si 2 6 A1 2 (OH) 4 + Ca H 2 (C0 3 ) 2 

Anorthite. Kaolin. 

The nature of the change may be more easily understood 
by comparing the constitutional formulae which have been 
proposed for the two substances. 

n j SiO.O A10.A10 f SiO.O A10.0H 2 

1 SiO.O CaO t SiO.O A10.0H 2 

Anorthite. Kaolin. 

It will be seen that, if these are correct, the change could 
be accomplished by the transference of an atom of 
aluminium to the place occupied by the calcium the 
latter being eliminated by the action of C0 2 and subse- 
quent hydration thus 




The composition of orthoclase differs from that of 
anorthite and it does not undergo change so readily. It 
is one of the most insoluble of rock-forming minerals, but 



CHEMISTEY OF SOILS 129 

is ultimately resolved into kaolin, potassium carbonate and 
silica, as shown in the following equation 

2KAlSi 3 8 +C0 2 -f 2H 2 = Si 2 5 Al 2 (OH)4+K 2 C03+4Si0 2 

Orthoclase. Kaolin. 

The silica is liberated in a highly hydrated, and possibly 
at first in a soluble, condition, but the water with which 
it is united is not shown in the equation because there is 
no means of knowing how much is present under different 
conditions. 

The relation between orthoclase and kaolin is not, per- 
haps, so readily apparent from the formulae but may be 
indicated thus 

f SiO.O A10.Si0 2 ( SiO.O A10.0H 2 

1 SiO.OK l SiO.O A10.0H 2 

Orthoclase. Kaolin. 

It will be seen that a silicate of the same type as kaolin 
would be produced by elimination of the Si0 2 group and 
hydration of the alumina ; the further replacement of the 
potash by alumina from another molecule and the break up 
of the latter would complete the change. This may be 
illustrated as follows : 

| SiO.OAl 0.(SiO~ 2 

I SiO 

(SiO 

( SiO 

Of the intermediate stages of the reaction it is impos- 
sible to speak with any certainty. Only a very small 
proportion of the potash is ever found to be soluble in 
water, but the fact is probably accounted for by the 
phenomena of absorption (p. 103). All the potash that 
is soluble in dilute acids has probably been reduced 




130 SOILS AND MANUBES 

to that condition by complete decomposition of the 
felspar. That portion which is soluble in concen- 
trated acids, but insoluble in dilute acids, is almost 
certainly a product of partial decomposition, and that 
which is not attacked by concentrated acids consists of 
the original or only very slightly altered mineral. Dilute 
and concentrated acids are, of course, relative terms, and 
it is impossible to distinguish sharply between the products 
of complete and partial decomposition by this means. 
Also, the amount of potash that can be extracted from soil 
by acid of any strength depends, to some extent, upon the 
temperature and time of extraction ; there appears to 
be no absolute finality about it. Even after prolonged 
extraction, renewed application of the solvent will gener- 
ally extract a little more. 

The presence of lime in the soil favours the liberation 
of potash. Three hypotheses are proposed in explanation 
of the fact, any or all of which may be correct. 

1. The lime, by reason of its affinity for silicic acid, 
favours the splitting off of Si0 2 with which it unites form- 
ing calcium silicate, thus 

H 2 Si0 3 + CaC0 3 = CaSi0 3 + H 2 + C0 2 

2. The lime acts simply as a carrier of C0 2 , thus 

2 KA1 Si 3 8 + CaH 2 (C0 3 ) 2 + H 2 = 

Si 2 5 A1 2 (OH) 4 + K 2 C0 8 + 4 Si0 2 + CaC0 3 

The first of these two reactions may be held to involve 
the second, and they may be represented jointly thus 

2 KA1 Si 3 8 + Ca C0 3 + 2 H 2 = 

Si 2 5 Al a (OH) 4 + K 2 C0 3 + CaSi0 3 + 3 Si0 2 

3. The lime simply displaces the potash, thus 

2 KA1 Si 3 8 + Ca C0 3 = CaAl 2 (Si 3 8 ) 2 + K 2 C0 3 



CHEMISTRY OF SOILS 131 

Oxidation and Reduction. The processes of oxidation 
and reduction are chiefly confined to the organic matter 
and minerals containing compounds of iron and man- 
ganese. Oxidation may be considered complete when all 
the iron is converted into the ferric state represented by 
the oxide Fe 2 3 . 

The oxidation of iron pyrites, sometimes found in soils 
derived from slates .and shales, may be shown thus 

2 FeS 2 + 15 + H 2 = Fe 2 (S0 4 ) 3 + H 2 S0 4 

It takes place much more rapidly in the presence of lime 
which combines with the acid thus 

2 FeS 2 + 4 CuC0 8 +150 = Fe 2 3 + 4 CaS0 4 + 4 C0 2 

Pyrites. Ferric oxide. 

Magnetite and ferrous compounds, commonly present in 
the minerals of the basaltic rocks, readily undergo oxida- 
tion. , , 
2Fe 3 4 + = 3Fe 2 3 

Magnetite. 

2 FeO + = Fe 2 8 
Ferrous oxide. Ferric oxide. 

Ferric oxide is easily formed and also easily reduced 
again to the ferrous state. It therefore acts as an oxidising 
agent or carrier of oxygen and produces a stimulating effect 
upon the soil. It accelerates the oxidation of organic 
matter and causes the nitrogen and other substances con- 
tained in it to become more rapidly available to plants. 
The presence of ferric oxide and organic matter in soils, 
therefore, generally indicates a high degree of fertility. 

There is, however, another side to the question. The 
oxidation of organic matter by ferric oxide involves the 
reduction of the latter, and, if the conditions do not admit 
of rapid re-oxidation, the ferrous compounds produced 
have a deleterious effect upon the grow,th of plants. la 

K 2 



132 SOILS AND MANUBES 

practice, the readiest means of oxidising the soil is to 
fallow it ; in fact, the beneficial effects of fallow are 
largely due to the processes of oxidation which it promotes. 
It is not without interest to note in passing that the word 
" fallow " is derived from the Saxon fealo a red or yellow 
colour and originally meant to redden the land, i.e., to 
oxidise the compounds of iron in the soil. 

Compounds of manganese are not very common in soils, 
but in some cases they occur in considerable quantities. 

They undergo oxidation and reduction like those of iron, 
but perhaps more readily, and produce similar effects. 

Dehydration. The hydrates of iron, alumina and silica, 
to which reference has been made in several places, when 
freshly precipitated, are obtained in a gelatinous condition. 
They consist of the oxides united with large but indefinite 
quantities of water. Much of the water, however, is so 
loosely united that it evaporates on exposure to dry air at 
ordinary temperatures, and the dry compounds of iron 
and alumina finally obtained are definite hydrates, corre- 
sponding to known salts, from which the remaining water 
can be expelled only by heat, thus 

2 A1(OH) 8 = A1 2 3 + 3 H 2 
2 Fe(OH) 3 = Fe 2 3 + 3 H 2 

The hydrates of silica corresponding to known salts con- 
tinue to lose water at ordinary temperatures. 

Si(OH) 4 = Si0 2 + 2 H 2 
SiO(OH) 2 = Si0 2 + H 2 

The gelatinous hydrates of indefinite composition are 
produced in the soil by the decomposition of minerals and 
other changes. On drying they shrink greatly in volume 
and become much leps readily soluble in acids or alkalis. 

Other Reactions. The phosphatic minerals of the soil 
probably consist chiefly of tricalcic phosphate Ca 3 



CHEMISTRY OF SOILS 133 

in a crystalline or an amorphous state. They are insoluble 
in water, and more or less difficultly soluble in dilute acids 
according to the strength, temperature, etc. It is pos- 
sible that in many cases the tricalcic phosphates do not 
undergo any chemical changes but gradually become more 
readily available to plants merely as a result of mechanical 
pulverisation. Any transient acidity of the soil would tend 
to convert the tricalcic phosphate into the soluble mono- 
calcic, CaH 4 (P0 4 ) 2 ;form. The phosphate, however, cannot 
long continue to exist in that state ; on contact with lime 
it is reprecipitated as dicalcic or tricalcic phosphate 

CaH 4 (P0 4 ) 2 + CaC0 3 = 2 CaHP0 4 + C0 2 + H 2 

Monocalcic Dicalcic 

phosphate. phosphate. 

CaH 4 (P0 4 ) 2 + 2 CaC0 3 = Ca 3 (P0 4 ) 2 + 2 C0 2 -+ 2H 2 

Monocalcic Tricalcic 

phosphate. phosphate. 

On contact with hydrates of iron or alumina this soluble 
phosphate would be precipitated as phosphates of these 
bases. 

3 CaH 4 (P0 4 ) 2 + 4 Fe (OH) 3 = 4 Fe P0 4 + Ca 8 (P0 4 ) 2 +12 H 2 

Monocalcic Ferric Tricalcic 

phosphate. phosphate. phosphate. 

3 CaH 4 (P0 4 ) 2 + 4 Al (OH) 3 = 4 Al P0 4 + Ca 3 (P0 4 ) 2 + 12 H 2 

Precipitated phosphate of lime is much more readily 
soluble in dilute acids than the crystalline mineral, and 
its solubility is probably not greatly diminished on drying. 
The phosphates of iron and alumina, even in the gelatinous 
hydrated condition in which they appear when freshly 
precipitated, are insoluble in dilute acetic and some other 
acids, but are probably available to plants. They are 
rendered much more difficultly soluble on drying, and 
probably they become gradually non-available. It is very 
desirable, therefore, that soluble phosphates should be 



134 SOILS AND MANURES 

precipitated as phosphates of lime rather than as phos- 
phates of iron and alumina. If the soil remained in an 
acid condition, precipitation would not take place ; on the 
other hand, if the soil contained enough lime to maintain 
a neutral condition, very little soluble phosphate would 
be formed. Quite perceptible amounts of phosphate are, 
however, dissolved by water highly charged with carbonic 
acid 

Ca 3 (P0 4 ) 2 + 4 C0 2 + 4 H 2 = CaH 4 (P0 4 ) 2 + 2 CaH 2 (C0 3 ) 2 

and the more dilute solutions in the soil have a similar if 
slower effect. Small or infinitesimal quantities of phos- 
phate are thus continually dissolved, and, in the absence 
of lime, may be reprecipitated as phosphates of iron and 
alumina. The gradual conversion of phosphate of lime 
into phosphate of iron by prolonged action of carbonic acid 
and water has actually been observed to take place, and, 
in time, it must result in a marked reduction of the fertility 
of the soil. 

Ca 3 (P0 4 ) 2 + 2 Fe (OH) 3 +6 C0 2 = 2 Fe P0 4 + 3 Ca H 2 (C0 3 ) 2 

It is possible that in the presence of an excess of lime 
the reverse action may occur ; phosphates of iron and 
alumina, if present, may be slowly converted into phos- 
phate of lime, thus 



This action may be compared with the displacement of 
potash from silicates by lime (p. 130), and with the general 
tendency of lime to combine with the acid radicles and 
set free the bases of soluble salts in the soil. 

The Air of Soils. The atmospheric air permeates the 
soil and fills the interstitial space, i.e., that portion of the 
total volume not occupied by solid particles and water. 
The vast extent of internal surface of the soil causes the 



CHEMISTEY OF SOILS 135 

gases to undergo a process of densification ; water vapour 
is condensed, ammonia is absorbed and the oxygen rendered 
much more active. The gases are not, however, perma- 
nently occluded ; a constant exchange takes place between 
the air of the soil and that of the atmosphere by the process 
of diffusion ; those which are absorbed are replenished, and 
those which are produced by chemical changes in the soil- 
chiefly carbon dioxide are diffused. 

Soils of finer texture present a greater internal surface 
and have, therefore, a greater power of absorbing atmos- 
pheric gases, but the same conditions tend to retard diffu- 
sion and cause the soil to retain more water. Under natural 
conditions, therefore, the air finds much readier access 
to soils of coarser texture. 

On marshy lands, in which the pores are permanently 
blocked with water, carburetted hydrogen, carbon mon- 
oxide, etc., are produced by the decomposition of organic 
matter, but under the conditions of rapid oxidation, which 
obtain in cultivated land, these gases are not formed, or, 
if formed, are immediately oxidised, and are never found 
in the soil gases. 

The air of the soil, therefore, contains the same gases 
that are found in the atmosphere, but they are often present 
in very different proportions. In general, the air of the 
soil contains less oxygen and more carbon dioxide. The 
oxygen is withdrawn by the oxidation of minerals and 
organic matter, and carbon dioxide is produced. The pro- 
portion of nitrogen by volume is not greatly affected by 
these changes because carbon dioxide occupies the same 
volume as the oxygen consumed. For example, the air of 
a forest soil was found to contain 0'93 per cent, of carbon 
dioxide and only 19 '5 per cent, of oxygen. The oxidation 
of organic matter is greatly accelerated by the action of 
micro-organisms and under conditions favourable to their 
growth chiefly moisture, warmth and a plentiful supply 



136 



SOILS AND MANUEES 



of oxygen takes place very rapidly. The proportion of 
oxygen in the air of the soil may thus be reduced to less 
than half that found in the atmosphere, i.e., to about 
10 per cent., and the proportion of carbon dioxide corre- 
spondingly increased. 

The proportion of carbon dioxide found in the air of soils 
under different conditions was as follows : 



Carbon Dioxide. 



The atmosphere . 
Sandy forest subsoil 
Loamy forest soil 
Arable soil 
Vineyard soil 
Pastureland soil . 
Sandy soil freshly manured (dry weather) . 
' , (wet ) . 



Per cent. 

04 

38 

1-24 

1-30 

1-46 

2-70 

3-33 

14-13 



The composition of the air of sandy soils generally re- 
sembles that of the atmosphere more closely than does that 
of other soils, because, owing to the rapidity with which 
oxidation takes places in the first, they generally contain 
but little organic matter. For the same reason, i.e., the 
rapidity of oxidation, when they contain much organic 
matter, e.f/., when freshly manured, a larger proportion of 
carbon dioxide is often found in the air of sandy soils than 
in any other. The rapidity with which organic matter is 
oxidised in sandy soils results from their open texture 
which promotes rapid diffusion, and, to a lefss extent, from 
the fact that they are generally warmer. On the other 
hand, the comparative dryness of sandy soils -tends to 
retard the action of micro-organisms, and the progress of 
the oxidation is, therefore, a,s a rule, greatly accelerated by 
wetting. 



CHAPTER V 

BIOLOGY OF SOILS 

The organisms. The organisms of the soil, with which 
the agriculturist is chiefly concerned, belong to the 
simplest order of plants. They are known to botanists as 
Thallophytae, and are divided by them into two groups, 
called respectively Algae and Fungi. The former contain 
chlorophyll and, often, other colouring matters as well. 
The latter contain no chlorophyll, and are usually white or 
colourless, but, when seen in mass, some of them exhibit 
red, yellow, green or bluish colours. Both groups are 
subject to minute botanical classification, but for pre- 
sent purposes the algse need not be subdivided at all, and 
it will be sufficient to divide the fungi into moulds and 
bacteria. From this p.oint of view the bacteria are by far 
the most important both numerically and functionally. 

Alga. These are simple cellular plants adapted for 
growth in moist places or in water. The sea-weeds belong 
to this group, but the forms present in the soil are of 
microscopic size. They are distinguished from the fungi 
by the presence of chlorophyll, which enables them to 
decompose carbon dioxide and obtain their carbon from 
the air. Sunlight is, therefore, necessary for their growth, 
and the fact implies that their sphere of action must be 
confined to the surface of the soil. 

It is believed that certain forms are able to utilise the 
free nitrogen of the air to form organic compounds, by 
the decomposition of which, after the death of the 
organism, the nitrogen is rendered available to higher 
plants. This is probably one of the sources of the com- 



138 SOILS AND M ANUSES 

bined nitrogen which accumulates in soils. The property 
of fixing and assimilating the free nitrogen of the air is 
shared by some of the soil bacteria. 

Moulds. Moulds are usually very abundant in cultivated 
soils. They are either parasitic or saprophitic in habit, 
and are often found closely associated with the roots of 
higher plants. It has been held that this association is 
of a symbiotic nature, but whether there is any true organic 
union or not, the effects are highly beneficial. As inde- 
pendent saprophytes they hasten the decomposition of 
organic matter probably by the action of enz ernes or 
soluble ferments and absorb the products. They grow 
with extraordinary rapidity ; the soft cellular tissues of 
which they are composed quickly decompose again, and 
the nitrogen and other constituents become available for 
the growth of higher plants. It is also possible that by 
the action of moulds upon organic matter in the soil, the 
products of decomposition may be, to some extent, 
rendered directly available to the higher plants, i.e., with- 
out being first absorbed by the moulds. This is certainly 
accomplished by bacteria and probably also by yeasts 
a group of fungi only slightly differentiated from moulds 
on the one hand, and from bacteria on the other. 

BACTERIA. 

Reproduction. Bacteria are present in the soil in 
enormous numbers, variously estimated 1 at from half a 
million to one million per gram. They multiply by simple 
fission. The process has been observed to take place, under 
favourable conditions, in about twenty minutes. It has 
been calculated that sixteen millions might be produced 
from a single cell in twenty-four hours. They also form 
spores. These are able to resist extremes of temperature, 

1 In some cases much larger numbers have been observed. 



BIOLOGY OF SOILS 139 

desiccation and other unfavourable conditions which would 
be fatal to the organism in its more active state. Under 
favourable conditions, the spores germinate and the 
organism is reproduced in a state of vital activity. 

Appearance. Bacteria can be propagated by growth in 
a suitable medium, and can often be recognised by the 
appearance of the cultures, whiclTsomewhat resemble that 
of mould growths but are generally denser. Individu- 
ally they can be distinguished only under high powers of 
the microscope, and are generally classified according to 
their shape as micrococci and bacilli. The former includes 
all the spherical and oval forms, and the latter the more 
elongated, rod-like and filamentous forms, but the line 
of division is not a sharp one. The micrococci are often 
associated in pairs, groups of four, or multiples of four, 
arranged in chains or scattered indefinitely. Of the bacilli 
some are straight, others are curved like a comma, doubly 
curved like a corkscrew, and sometimes intertwined. The 
names given by bacteriologists to these various types need 
not now be considered. 

Food. In general, bacteria require the same mineral 
elements as other plants, but are adapted for growth in a 
very dilute medium. As they contain no chlorophyll they 
are unable 1 to decompose carbon dioxide but obtain their 
carbon chiefly from carbohydrates, e.g., sugar and other 
similar compounds. Nitrogen, as a rule, is derived from 
organic compounds, though some bacteria can obtain it 
from ammonium salts and probably also from nitrates and 
the free nitrogen of the air. Some are parasitic and 
others saprophitic, i.e., some grow only on a living host 
usually a particular organism and not infrequently a 



1 Kecent researches have shown that, under certain conditions, 
some bacteria can obtain carbon from carbonates and from carbon 
dioxide, notwithstanding the absence of chlorophyll. 



140 SOILS AND MANUKES 

particular part of it others only on decaying organic 
matter. 

Some bacteria require oxygen, others grow only in its 
absence, while others again are indifferent but produce 
very different results according to the circumstances. The 
majority grow best in a neutral medium, but some do better 
when it is faintly acid or alkaline. A few only can with- 
stand a high degree of acidity or alkalinity ; the others 
are, therefore, liable ,to inhibit their own action by the 
acid or alkaline character of their products. 

Temperature. For each type of organism there is a 
particular temperature at which it grows best, and also 
a maximum and a minimum temperature beyond which 
it does not grow at all. The limits of variation in these 
respects are very wide; most types flourish between 
20 C. and 30 C., but some of the soil bacteria found near 
the surface do not develop at temperatures below 60 C. 
In all cases vitality ceases at a temperature below 100 C., 
but some form spores which can withstand a temperature 
considerably higher. 

Antiseptics. Certain chemicals which retard or inhibit 
the growth of bacteria and other organisms are called anti- 
septics. Powerful oxidising agents such as chlorine, per- 
manganate of potash, and mineral poisons, e.g., salts of 
mercury, copper, silver, etc., are amongst the most effec- 
tive. Carbolic acid, sulphur dioxide, and some other sub 
stances are also used as disinfectants on account of their 
antiseptic action. Salicylic acid, formic aldehyde, borax 
and common salt have each a similar but much milder 
effect, and are used as preservatives for food. 

Sterilisation. When all the living organisms in a soil, 
or other substance, have been destroyed, either by exposure 
to high temperatures or by treatment with antiseptics, the 
substance is said to be sterilised. Since, however, bacteria 
are universally distributed in the air, water and all around, 



BIOLOGY OF SOILS 141 

the sterile condition cannot be maintained unless the re- 
entrance of bacteria is prevented by special precautions. 

Chemical Changes due to Bacteria. The chemical 
changes due to bacteria are too numerous to recount, and 
too complex to be explained here. In many cases the 
nature of the change is either imperfectly understood 
or totally unknown. The general tendency, however, is 
to decompose compounds of complex molecular structure 
and produce simpler substances by splitting off groups 
or even single elements, and by hydrolysis, oxidation, 
reduction, etc. By the researches of Pasteur and 
others many diseases of animals, e.g., tuberculosis, 
cholera, etc., have been traced to the action of bacteria ; the 
souring of milk, fermentation of urine, and many other 
familiar changes are due to the same cause. In some 
instances the specific organisms which produce these various 
effects have been isolated and identified ; the pathogenic 
forms have received the most attention, and, as there are 
more workers in that field, they are generally better known. 

Bacteria also take part in the formation of organic 
matter in soils. They are found on bare rock surfaces, 
at mountain tops and elsewhere, under conditions which 
show that they must derive nourishment from the atmos- 
phere. Notwithstanding the absence of chlorophyll, it is 
believed that certain forms can obtain carbon from carbon 
dioxide, and can assimilate free nitrogen or obtain that 
element from ammonia. Small quantities of organic 
matter are thus produced, and are generally found adhering 
to the particles of disintegrated rocks in the early stages 
of soil formation. The most important of the changes due 
to bacteria in the soil, however, are the decomposition of 
organic matter either nitrification or putrefactive decay 
and the fixation of free nitrogen, directly like algse, or 
indirectly by symbiotic association with higher plants, 
chiefly, if not exclusively, belonging to the order leguminosae^ 



142 



SOILS AND MANURES 



Fixation of Free Nitrogen by Leguminous Plants. 
From time immemorial it has been customary to take wheat 
after clover because it was found that better crops were 
obtained than in any other order of rotation. It has also 




FIG. 17. 



been known for a long time that, when used as green 
manures, clover vetches and lupines gave better results 
than most other crops. More recently it was discovered 
that, although these plants contain a very large amount 
of nitrogen (p. 169), the proportion of nitrogen in the soil 



BIOLOGY OF SOILS 143 

is not diminished but increased by their growth. This applies 
more or less to all plants belonging to the order leguminosse. 

HeUriegeVs Discovery. No satisfactory explanation of 
these remarkable facts was forthcoming, however, until 
the year 1886, when Hellriegel and Wilfarth showed that, 
under certain conditions, leguminous plants can assimi- 
late the free nitrogen of the air and grow in soil devoid 
of compounds of nitrogen, from which plants usually obtain 
their supplies of that element. 

This peculiar property they traced to the development of 
warts or nodules on the roots of the plants (Fig. 17), and 
showed that these are produced by bacteria which enter the 
plant and are the cause of the fixation of nitrogen. When 
grown in sterilised soil the warts or nodules are not formed, 
and in the absence of compounds of nitrogen development 
of the plants is arrested. 

Sand Cultures. The fact can be demonstrated by sand 
culture experiments similar to those previously described 
(p. 7). When the sand has been prepared and mixed 
with the necessary mineral ingredients of plant food com- 
pounds of nitrogen being strictly excluded it is put into 
pots and sterilised by heat. When cold the sand is satu- 
rated with sterilised water and the seed of any leguminous 
plant peas, clover, vetches, etc. deposited in it. The pots 
are then covered with bell glasses and kept at a suitable 
temperature. In due time the seeds germinate, but the plants 
soon languish for lack of nitrogen and die. But, if the sand 
in one of the pots be impregnated 1 with soil bacteria and 
a small quantity of nitrate of soda is added 2 to another, a 
very different result will be produced (Figs. 18 and 19). 

1 This may be done by sprinkling a few grains of moist earth from 
an ordinary soil on the surface of the sand, or by shaking up a handful 
of ordinary soil with water, and, when it has settled, decanting off the 
liquid and adding it to the sand under experiment. 

' 2 Goldim?. 



144 SOILS AND MANUEES 

In both cases the plants continue to grow, and may, in 
time, bear seed. If they are now removed from the sand 
and examined it will be seen the roots of the plants grown 
in sand impregnated with soil bacteria are covered with 




FIG. 18. (1) No Combined Nitrogen ; (2) Inoculated 
with Crushed Nodule of a Bean. 



nodules, but none will be found on the roots of those grown 
in sterilised soil. Thus it is shown that the presence of soil 
bacteria causes the development of nodules on the roots 
and makes the plants independent of compounds of nitro- 
gen in the soil. In the absence of such bacteria no nodules 
are formed, and leguminous plants must, like others, 



BIOLOGY OF SOILS 



145 



obtain their nitrogen from suitable compounds in the soil. 
If neither source of nitrogen is available the plants die; 




FIG. 19. Beans grown with (1) Nitrogen from air ; (2) Nitrogen 
from soil ; (3) No nitrogen. 

if both are available the plants do very much better than 
on either alone. Many farmers, though well aware of the 
capacity of leguminous plants for obtaining nitrogen from 

S.M. L 



146 SOILS AND. MANURES 

the air, consider that a light dressing of nitrogenous, 
manure is highly beneficial to these crops. 

The Seat of Fixation. The seat of fixation of the 
nitrogen has been a matter of some dispute. Some 
observers have declared it to be in the leaves and others 
in the root of the leguminous plant, but there appears to 
be little room for doubt that it is in the nodules themselves 
that fixation actually takes place. It has been found that 
the process is arrested when the nodules are submerged 
in water. The nodular cells are in communication with 
those of the root proper by a regular system of conducting 
vessels, and they exhibit signs of intense metabolic activity. 
There is thus, apparently, a true organic union between 
the bacteria and the higher plant, and the opinion is widely 
held, if not universally accepted, that the stimulating effect 
of the symbiotic association is the true cause of fixation. 

Inoculation of Soils. The discovery of the fixation of 
free nitrogen by leguminous plants and the cause thereof 
naturally gave rise to somewhat inflated ideas as to the 
practical benefits likely to be derived from it. It was 
anticipated that by inoculating the soil with bacteria, 
larger crops would be obtained, a remedy would be found 
for clover sickness, and that it might become possible to 
grow leguminous crops on soils hitherto regarded as not 
naturally well adapted for the purpose. The method of 
inoculation first proposed was to top dress the land with 
soil taken from another field on which leguminous crops 
were known to flourish. A great many experiments on 
these lines have been tried, and some of them gave positive 
results. Those with which the writer was concerned did 
not. Subsequent investigations have shown that though a 
certain improvement may sometimes be effected in this 
way, the beneficial effects are, as a rule, too small to be 
of practical importance. 

Later it was claimed that the bacillus radicicola the 



BIOLOGY OF SOILS 



147 



FIG. 20. Organism from Boot-nodule of Bean. 1 




FIG. 21. Nodule Organism Grown in Artificial Culture. 
Inoculating Material for Peas. 1 



Golding. 



148 SOILS AND MANUKES 

specific organism to which fixation is due had been iso- 
lated, and preparations purporting to be pure cultures of 
the microbe were placed on the market under the name 
of " mtragin." The results obtained were so disappointing 
that the sale was soon discontinued. The name, however, 
remains, and has been applied to all subsequent prepara- 
tions of a similar kind. 

The prophetic words of the late chemist to the Highland 
and Agricultural Society of Scotland on this subject were 
amply justified at the time, and would seem to preclude 
any very sanguine expectations for the future. He pointed 
out that the bacteria, which are the cause of the develop- 
ment of nodules on the roots of leguminous plants, must 
be plentifully distributed in all soils capable of bearing 
good leguminous crops, and that the addition of a few 
hundreds or thousands could not be expected to make any 
very great difference. Also that, if a soil were deficient in 
the bacteria, it would probably be because the conditions 
were not suitable for their growth ; it would, therefore, 
be useless to add more until the adverse conditions had 
been discovered and removed. He thought that, if the con- 
ditions were suitable for the growth of leguminous crops, 
the bacteria would naturally multiply so rapidly that in- 
oculation would be superfluous. 

When the soil is inoculated with the juice expressed from 
the root nodules a favourable effect is generally produced 
on the growth of plants of the same kind, but not on others. 
It appears that each kind of leguminous crop requires a 
particular kind of microbe. It is believed that most of 
these micro-organisms belong to the same species, but 
they become so modified by association with particular 
kinds of plants that they are unable to form nodules on 
others. If the micro-organisms from one kind of plant 
do become associated with another they become completely 
adapted to their new host and will no longer form nodules 



BIOLOGY OF SOILS 149 

on the kind of plants from which they were originally 
derived. It is in this adaptability, or capacity for modifi- 
cation, that the greatest hope of future practical develop- 
ments lies. The bacteria may be modified by cultivation, 
as higher plants have been, and forms or varieties suitable 
for purposes of practical utility may be produced. 

The bacteria required for the commoner kinds of legu- 
minous crops, e.cj., clovers, vetches, etc., are probably pre- 
sent, in active form, in all the cultivated soils in this 
country that are adapted for the growth of these plants. 
It is by no means certain, however, that the microbes re- 
quired for other plants are so plentiful. Should any diffi- 
culty be experienced in raising a leguminous crop on a 
soil on which it has not previously been grown, it may 
possibly be due to absence of the necessary bacteria. If 
the difficulty be not due to other obvious causes, such as 
lack of lime, phosphates, etc., inoculation of the soil should 
be tried. In some cases it has proved useful. 

As a means of stimulating the growth of crops on soils 
which already contain the necessary bacteria, the outlook 
is, perhaps, less promising. Numerous experiments are, 
however, being carried on, and developments of practical 
importance are hoped for in the future. The following 
results were obtained in the soil inoculation experiments 
at Eothamsted in 1907 : 

EFFECT OF INOCULATING THE SOIL UPON TUB GROWTH OF LEGUMINOUS 
PLANTS. PRODUCE OF KBD CLOVER (HAY), 1907. 



Plot. 


Soil Inoculated with 


Produce, 1st aud 
2nd Crops. 






cwt. 


A. 


Hiltner's preparation from Munich 


66-1 


B. 


Moore's preparation from the United States . 


57-4 


C. 


Soil from a field which had carried red clover 






in 1904 


59-0 


D. 


Left uninoculated ...... 


56-6 




<D <U 
5H f* 

P -2 
33 

o o 

02 02 



rrt ^ 

ll 



CC 02 







^; o Q 



o o 



O 
Ifl 02 





33 



w o 

o o 



C 0) 

03 OJ 



BIOLOGY OF SOILS 151 

The crop from the uninoculated plot must be reckoned 
a fair one. The plants were healthy, and the roots were 
covered with nodules. All the inoculated plots have given 
a larger yield. With Hiltner's preparation the increase 
is distinct and substantial. In the other two plots the 
increase is so small as to come within the ordinary varia- 
tions in experiments of this kind, and cannot be confidently 
attributed to the inoculation. Even in the case of Hiltner's 
preparation, it should be remembered that the experiment 
has lasted only two years. The results must be confirmed 
before it can be assumed that similar benefits will gene- 
rally follow such treatment. 

Fixation by Non-leguminous Plants. It is still uncer- 
tain whether plants belonging to orders other than the 
leguminosse can assimilate nitrogen in its elementary form. 
Certain experiments seem to show that such is the case, 
but they are not considered entirely conclusive. There 
is, at least, no satisfactory evidence to show that any of 
them possess this property in anything like the same degree 
as the leguminous plants. On the other hand, there is 
much negative evidence to the contrary. 

Alinite. The name alinite has been given to another 
bacterial preparation for supplying nitrogen to crops. It 
is not now believed that the organisms in this kind of 
culture have any power of fixing the free nitrogen, but they 
probably accelerate the decomposition of nitrogenous 
organic compounds. It has been found to give positive 
results on soils rich in humus. 

DESTRUCTION OF ORGANIC MATTER. 

Decay and Putrefaction. When organic matter is 
burned it undergoes oxidation, and the elements of which 
it is composed except nitrogen are liberated as oxides. 
It is also destroyed when heated in the absence of oxygen ; 



152 SOILS AND MANUBES 

corresponding compounds of hydrogen are formed, together 
with some other substances of more complex composition. 
Similar changes are produced in the soil by the action 
of micro-organisms. The changes take place more slowly 
and pass through numerous intermediate stages, but the 
ultimate result is much the same. In the presence of 
oxygen all the elements including nitrogen are converted 
into oxides 

C0 2 ; H 2 0; S0 3 ; N 2 5 

In the absence of oxygen the corresponding compounds 
of hydrogen 

CH 4 ; OH 2 ; SH 2 ; NH 3 

are produced. The acid oxides S0 3 and N 2 5 combine with 
bases to form sulphates and nitrates. The process by 
which they are formed is called nitrification. Slow decom- 
position in the absence of oxygen is called putrefaction. 
Strong, disgusting odours are generally evolved, especially 
if much animal matter is present. 

Artificial Production of Nitrates. Potassium nitrate was 
artificially produced from nitrogenous organic matter long 
before it was known that the process of nitrification was 
due to the action of micro-organisms. Waste organic 
matter of all kinds the richer in nitrogen the better was 
mixed with earth and lime and moistened with liquid 
manure to promote fermentation. After the lapse of a 
considerable time the heap was lixiviated with water to 
dissolve out the calcium nitrate which had been formed, 
and potassium carbonate was added to the solution. These 
two substances react upon each other, producing calcium 
carbonate and potassium nitrate thus 

Ca(N0 3 ) 2 + K 2 C0 8 = CaC0 3 + 2 KN0 3 

Calcium Potassium Calcium Potassium 
nitrate. carbonate, carbonate. nitrate. 



BIOLOGY OP SOILS 153 

Calcium carbonate, being insoluble, was precipitated, 
and the potassium nitrate which remained in solution was 
then crystallised out. The process is necessarily a slow 
one, and, since the discovery of natural deposits of nitrates, 
is no longer required. 

Composts. For agricultural purposes it is generally 
better to bury the organic matter in the soil and allow nitri- 
fication to take place there. The process above described 
is, however, still resorted to as a means of dealing with 
substances which cannot be directly incorporated with the 
soil. The nitrifying heaps are known as composts, and, when 
the process is complete, the whole of the material is spread 
on the land without converting the nitrates into other forms. 

The practice of composting the farmyard manure, i.e., 
of mixing it with two or three times its weight of ordinary 
soil, usually without lime, before applying it to the land, 
is ^greatly favoured by some farmers. It hastens the action 
of the manure, and, as the farmers say, makes it go further, 
i.e., makes it more easy to spread uniformly over a 
large area. Turning farmyard manure is a laborious and, 
as will be shown later, a wasteful process. It promotes 
rapid fermentation and loss of nitrogen results. The 
admixture of soil would tend to minimise the loss, but the 
operation cannot be recommended for general use. 

NITRIFICATION. 

Nature of the Change. The conversion of the nitrogen 
of organic compounds, more particularly of proteids, into 
nitric acid, by the action of micro-organisms, is a very 
complex process. It is essentially a process of oxidation, 
but probably hydrolytic and other changes are also 
involved. Carbon dioxide and water are produced. The 
change takes place in several successive stages marked by 
the formation of definite intermediate products, of which 
ammonia and nitrous acid are the most important. 



154 SOILS AND MANUBES 

Production of Ammonia from Proteids. The final stages 
are simple and well understood. The changes antecedent 
to the formation of ammonia are more obscure. It is 
tolerably certain that amide 1 bodies, such as glycocine, 
leucine, asparagine, tyrosine, cystine, etc., are formed in 
most if not in all cases. 

Humus probably contains bodies of this class as well as 
other compounds of nitrogen. They are always produced 
by the decomposition of proteids by purely chemical means, 
i.e., by acid or alkaline hydrolysis, and are called the 
primary dissociation products. They are not directly 
saponifiable, but ammonia is produced from them by the 
action both of formless and of organised ferments, prob- 
ably as a result of the complete or partial oxidation of the 
non-nitrogenous part. 

1 Amides, properly so called, are acids in which the acid hydroxyl 
is replaced by NH 2 , e.g., urea (carbamide) CO(NH. 2 )-2. In agricultural 
chemistry the term is often used to include compounds in which an 
alcoholic hydroxyl is replaced by NH 2 . Those mentioned in the text 
except asparagine, which belongs to both all belong to the latter 
type, and are more properly called amino-acids. The formulae 
ascribed to them are as follows : 

Glycocine, amino-acetic acid, 

>NH\ rR 
\CO / 

Leucine, isobutyl-amino-acetic acid, 

O 3 CH-CH 2 -CH 



Asparagine, amino-succinamide, 

O \?^ : ^> CH-CHo-CO-NH.> 
\LU / 

Tyrosine, oxyphenyl-amino-propionic-acid, 

O <^cc^ 3 / CH-CH, C H 4 OH 
Cystine, dithio-dialanine, 




PIG. 23. 




FIG. 24. 



156 SOILS AND MANUEES 

The production of ammonia from the proteids of anima] 
and vegetable matter in the soil is probably effected, 
directly and indirectly, by the action of moulds, as well 
as by many different types of bacteria. An organism 
known as bacillus mycoides, usually very abundant in 
arable soils, appears to be particularly active, and is com- 
monly credited with a large share of the work. It has been 
definitely ascertained that this microbe causes the decom- 
position both of proteids and amides, such as those 
mentioned above, with the production of ammonia. 

Production of Nitric Acid from Ammonia. The con- 
version of ammonia into nitric acid is due exclusively to 
the action of bacteria. It takes place in two distinct steps. 
First the ammonia is oxidised to nitrous acid, and then the 
latter is further oxidised to nitric acid. Each of these two 
steps is effected by a separate and particular type of 
organism, neither of which can perform the work of the 
other. According to Winogradsky, the nitrifying organisms 
obtained from soils in widely different localities are not 
identical, but each soil contains only one type of organism 
capable of oxidising ammonia to nitrous acid, and one 
capable of oxidising nitrous acid to nitric acid. 

The Nitric and Nitrous Ferments. The two kinds of 
microbe are quite distinct, and, when isolated, are easily 
distinguished by their appearance under the microscope. 
The nitrous ferment (Fig. 23) 1 is described as consisting 
of nearly spherical corpuscles, i.e., micrococci, of about 
a thousandth part of a millimetre diameter, and fairly 
uniform both in size and shape. The nitric ferment 
(Fig. 24) l is of a more cylindrical form, one dimension 
being two or three times greater than another, and is, 
therefore, generally classed with the bacilli. It is smaller 
than the nitrous ferment, its length being only about half 
the diameter of the latter. 

1 Winogradsky, Annales de 1'Institut Pasteur, 1891. 



BIOLOGY OF SOILS 157 

Simultaneous Action. Although the successive changes 
in the process are caused by different organisms, the latter 
are, for the most part, similarly influenced by like condi- 
tions. The changes, therefore, take place almost simul- 
taneously, and the intermediate compounds do not accumu- 
late. Accumulation of ammonium compounds has a 
peculiar effect upon both the nitrous and the nitric fer- 
ments. Within certain limits, the larger the proportion 
of ammonium compounds present the better for the deve- 
lopment of the nitrous ferment. But excessive quantities, 
i.e., more than about one part per thousand of neutral 
salts of ammonia, and less than half that quantity of 
alkaline ammonium carbonate, inhibit its action altogether. 
Much smaller quantities of ammonium salts inhibit the 
action of the nitric ferment, but as these are gradually 
converted into nitrous :acid by the action of the nitrous 
ferment, it resumes its activity and converts the nitrous 
into nitric acid. 

Habitat and Conditions of Growth. The soil is the 
natural habitat of the nitrifying bacteria. They are 
usually present in very large numbers, especially in soils 
moderately rich in organic matter. For this reason, and 
also because they require oxygen, they are much more 
abundant in the surface soil, i.e., that part of the soil 
which comes under cultivation, than in the subsoil. They 
are found at a greater depth in open, sandy soils, into 
which the air penetrates more freely, than in stiff clays. 
As a rule very few are found at a depth greater than 
3 feet, and none at all at a depth greater than 9 feet. 
The largest numbers are not, however, found actually on 
the surface, because they grow best in the dark strong 
sunlight destroys them and because the soil at the sur- 
face is often too dry. A plentiful supply of moisture is 
necessary for their growth, but excessive quantities hinder 
it by excluding the oxygen necessary for oxidation. It 



158 SOILS AND MANURES 

is probably owing to their extreme sensitiveness to des- 
iccation that they are not found in air. They are always 
present in water from wells and most other sources, but 
are, naturally, more numerous in sewage and water con- 
taining organic impurities. 

Pood. The nitrifying bacteria are adapted for growth 
in a very dilute medium. They require the usual mineral 
elements of plant food, viz., sulphates and phosphates of 
potash, magnesia and lime, but they can assimilate free 
nitrogen and obtain carbon from carbon dioxide, and are, 
therefore, independent of organic matter. 

Temperature. The most favourable temperature for the 
growth of nitrifying organisms is 37 C. Below 5 C. they 
develop very slowly, and very little nitric acid is, there- 
fore, formed in the soil during the cold season of the year. 
The maximum limit is about 50 -- 55 C. ; at higher tem- 
peratures the organisms are destroyed. 

Neutral Medium. They grow best in neutral or faintly 
alkaline solutions. The final products are acid, and, unless 
removed as they are formed, inhibit the action of the 
organisms. Alkaline hydrates and carbonates obviously 
cannot be employed to neutralise the acid, but the milder 
alkalinity of bicarbonates has a less unfavourable effect. 
Calcium carbonate is, however, by far the most suitable 
substance for the purpose. It is neutral and insoluble, 
but readily reacts with the nitric acid forming neutral 
soluble salts. To promote nitrification in this way is 
one of the most important functions of lime in the 
soil (p. 115). 

The Rate of Nitrification. So far as is known all kinds 
of nitrogenous organic matter undergo nitrification, but 
some are more susceptible than others. For example, 
wool, horn and fibrous tissues are much more resistant 
than blood, urine, cellular fungi, etc. The rate at which 
nitrification takes place in the soil, provided it is not 



BIOLOGY OF SOILS 159 

restricted by lack of moisture, oxygen, or any of the neces- 
sary conditions, depends mainly upon the temperature. 
It was found that about one part per million of nitric 
acid was produced daily in a soil containing 18 per cent, 
of humus and 0*3 per cent, of lime when kept at a tem- 
perature of 25 C. The addition of calcium carbonate or 
neutral potassium salts made very little difference, but 
when potassium carbonate or neutral potassium salts and 
lime were added together, the rate of nitrification was 
greatly increased. 

Denitrification. The presence of oxygen is necessary not 
only for the production of nitrates, but also for the con- 
tinuance of the nitrogen in that state. If the conditions 
prevailing in the soil are favourable to reduction, i.e., if 
it is " not in a state of active oxidation, a reverse change 
called denitrification takes place. The free element, and 
sometimes also oxides of nitrogen, are evolved and serious 
loss results. 

This destruction of nitrates has been attributed by some 
authorities simply to the action of reducing agents, par- 
ticularly organic matter. It has been observed to follow 
the application of very large quantities of farmyard 
manure, but it cannot occur to any extent when the soil 
is thoroughly aerated. 

The reduction of nitrates is, however, generally believed 
to be due in part, if not wholly, to the action of anaerobic 
bacteria. The activity of the nitrifying ferments is greatly 
reduced when the supply of oxygen is diminished, and a 
point is soon reached at which their action ceases alto- 
gether. The conditions are then favourable to the action 
of anaerobic organisms which are normally present in the 
.soil. It is known that many of these bacteria can reduce 
nitrates to nitrites, and even to nitrogen. 

The liberation of free nitrogen and gaseous oxides may 
be due to the direct action of the denitrifying organisms, 



160 SOILS AND MANUEES 

or it may be supposed that they merely cause reduction 
of nitrates to nitrites and ammonia, and that the libera- 
tion of the gases is due to the interaction of these com- 
pounds. Nitrous acid and ammonium nitrite are both very 
unstable ; the former breaks up into nitric oxide, nitric 
acid and water, and the latter into water and free nitrogen. 
There are difficulties in connection with such a theory, and 
the question is one for experiment rather than speculation. 

Whatever may be the true explanation, the facts point 
to the importance of maintaining the soil in a state of 
thorough oxidation. They show also that the use of ex- 
cessive quantities of farmyard manure may be actually 
injurious to the crops, and that it is inadvisable, in some 
cases, to combine the use of large dressings of farmyard 
manure and nitrates. 

Practical Importance. The capacity of soils to supply 
crops with nitrogen depends upon the process of nitrifi- 
cation, for it is by that means that the nitrogenous com- 
pounds are rendered available to plants. Nitrification has, 
therefore, a very close relation to productiveness. It takes 
place naturally on all cultivated soils, but can be acceler- 
ated by treatment which ensures suitable conditions. It 
is chiefly influenced by the temperature, and the supply of 
moisture, oxygen and lime. 

Good drainage has a favourable effect ; by removing 
excess of water, the temperature of the soil is raised and 
oxygen finds more ready access. 

Cultivation also favours nitrification by opening up the 
soil and exposing it to oxygen, but if carried on so as to un- 
duly promote evaporation it may have a contrary effect. The 
penetration of the roots of plants helps to loosen the soil 
and admit air, and nitrification generally proceeds faster 
in soils under crop than in bare soils under like conditions. 
Consolidation tends to exclude the oxygen, and on some 
soils may retard the process. 



BIOLOGY OF SOILS 161 

Liming has a good effect if the soil is markedly deficient 
in that element ; a very slight acidity arrests the process 
altogether. A comparatively small proportion of lime 
less than 1 per cent. is, however, sufficient for the pur- 
pose, and the addition of more produces no improvement. 
Fresh lime is soluble in water, and has an alkaline reaction ; 
it has, therefore, a tendency to arrest nitrification, but if 
applied in the back end of the year this effect is not noticeable, 
as it is soon converted into the neutral insoluble carbonate. 

Nitrification takes place very slowly during the cold 
winter months, but proceeds more rapidly as the soil 
becomes warmed by the summer heat. It takes place very 
fast during the month of July, and reaches a maximum in 
September (p. 104). There is thus a tendency for nitrates 
to accumulate in the soil towards the autumn. 

The Nitrogen of the Soil. Agricultural crops, except 
those which belong to the order leguminosae, do not obtain 
nitrogen directly from the air, but absorb it, like other 
elements, from compounds pre-existing in the soil. 

Nitrates are the most suitable compounds of nitrogen 
for the nourishment of plants. It has been shown by 
experiments that plants can assimilate nitrogen in the form 
of ammonia and other compounds, but these substances 
undergo nitrification in the soil very rapidly, and are rarely 
present in any considerable quantity. Under ordinary cir- 
cumstances, therefore, the available nitrogen consists 
almost entirely of nitrates. 

Unlike other elements of plant food, nitrogen is not 
originally a constituent of the parent substance of the soil. 
It is all derived, ultimately, from the atmosphere, (1) by 
the direct absorption of ammonia vapour ; (2) in the form 
of ammonia, nitrous and nitric acids, and organic com- 
pounds brought down by rain ; (3) by the action of algae 
and bacteria directly ; (4) by the action of bacteria in 
association with leguminous crops. 

S.M. M 



162 SOILS AND MANURES 

The nitrogen from all these sources undergoes nitrifica- 
tion more or less quickly according to the nature of the 
compounds, but the nitrates are quickly taken up by plants 
and converted into organic matter. The bulk of the nitro- 
gen is, therefore, present in the soil in an inactive form. 
It gradually becomes available as the organic matter 
decays, but the proportion present, in an available state, 
at any time is comparatively small. In this respect it is 
in exactly the same position as the other elements of plant 
food. 

But there is another reason why nitrates do not accumu- 
late in the soil. They are all very soluble in water, and are 
peculiarly liable to loss by drainage. The loss can be 
largely diminished by growing " catch crops " in the 
autumn. The nitrates formed during the summer are 
picked up, converted into organic compounds, and so 
preserved. The crops may be either fed off, or ploughed in 
green, in order to restore the nitrogen to the soil. 



CHAPTER VI 

FERTILITY 

Natural Productiveness. Fertility and infertility, as 
ordinarily applied to soils, are essentially relative terms. 
Soils are never absolutely barren unless rendered so by 
climatic conditions, the presence of injurious substances, 
or 'other accidental circumstances. Of soils which have 
been reclaimed and improved by mechanical treatment, 
i.e., draining, tilling, etc., some are naturally much more 
productive than others, and are said to be more fertile. 

The natural productiveness may be greatly increased 
or it may be diminished by systematic cultivation. In 
some cases, especially when land is scarce or dear, it is 
profitable to increase the productive capacity and to main- 
tain it at a point far above the natural fertility of the soil. 
When land is fairly cheap and naturally of a high degree 
of fertility, it may be more profitable to rely upon the 
natural resources of the soil. 

When the fertility of a soil has been diminished by re- 
moval of available plant foods, the soil is said to be 
exhausted. This may mean merely that the productive- 
ness has been reduced below the level at which it is 
normally maintained, and to which it may have been arti- 
ficially raised ; or it may mean that it has been reduced 
to a point below the natural capacity of the soil. In 
ordinary language, the soil may be said to be completely 
exhausted when the fertility has been so reduced that it 
ceases to give a profitable return for the labour employed 

M 2 



164 SOILS AND MANURES 

in cultivating it. It does not imply that the soil has been 
rendered absolutely barren by the removal of the last 
vestiges of available plant foods. Such a condition is prac- 
tically unattainable, and, if it were, it would not be per- 
manent. The conditions which originally gave rise to the 
formation of the soil are in constant operation, and avail- 
able plant foods are constantly being produced from the 
original sources. The reserves of plant foods in a non- 
available state are practically inexhaustible. 

The natural productiveness of soils is, of course, an 
indefinite term. It refers to the amount of produce that 
can be obtained from a given area of land when nothing 
in the shape of manure is added to it. This, however, 
depends very largely upon whether or not the produce is 
removed. Let it be supposed that the physical and bio- 
logical conditions of fertility are satisfied, and that pro- 
ductiveness is limited only by the supply of available plant 
foods. If the crops are not removed from the land, all 
the ingredients will be, in time, restored to it, and again 
become available for the growth of crops. During the 
interval occupied by the growth and decomposition of the 
plants, a further quantity of available plant food is formed 
from the original sources and is added to the ingredients of 
the previous crops. A larger growth is thus rendered pos- 
sible, and the productiveness of the soil is increased. 

The formation of available plant food in the soil, the 
abstraction of the same by growing crops, and the decom- 
position of the plants with the reconversion of the ingre- 
dients into the available state, constitutes a cycle of changes 
which might, theoretically, go on for ever without gain 
or loss, thus : 

Crops - > Humus 



Soil 



FEETILITY 165 

In practice there is both gain and loss, but the former 
exceeds the latter, and so, each time the cycle is completed, 
the capacity of the soil for the production of crops is 
increased. 

Effect of Removing Crops from the Land. The quantity 
of plant foods absorbed from the soil by a growing crop 
is considerably greater than the quantity that is converted 
into an available state in the same time, and, if the crops 
are removed from the land, the productiveness is neces- 
sarily diminished. It is obvious that a system of farming 
which tends to reduce the fertility of the soil must sooner 
or later come to a stop, and that, if farming is to be carried 
on .continuously, the system must be modified so as to 
maintain or, if possible, to increase the fertility. 

Maintenance of Fertility. There is thus an apparent 
dilemma. On the one hand, it is useless to grow crops unless 
they can be removed from the land ; on the other, if the 
crops are removed the fertility is reduced. But it is pos- 
sible to pursue a middle course, namely, to withdraw from 
the cycle, i.e., to permanently remove from the land only 
so much produce as corresponds to the quantity of plant 
food that becomes available in the same time. On cul- 
tivated land the change is greatly accelerated by tillage, 
and there is, therefore, a larger margin to draw upon. The 
amount of produce annually withdrawn from the cycle 
is reduced by subjecting the land to occasional or 
periodical fallow, by laying it down to grass, and by 
restoring a portion of the crops. To obtain a profitable 
return when the amount of produce that can be safely 
withdrawn is so limited is not a simple matter, and deserves 
more careful consideration. 

Fallow. Probably at one time it was customary to til] 
the land and to remove the crops until it ceased to give 
a profitable return for the labour expended upon it, and then 
it was neglected. When it was found that the fertility of 



166 SOILS AND MANUEES 

the land was restored after a time, owing to the re-accumu- 
lation of available plant food, it would be again put under 
cultivation. This is the fundamental principle of the 
modern system of fallowing. After several crops have 
been taken off the land it is allowed to rest for a period, 
in order that the available plant foods may re-accumulate. 
It is not, however, neglected, but is vigorously tilled in 
order to accelerate the chemical changes and to destroy 
weeds. The land is never allowed to become completely 
exhausted, and so a comparatively short period of fallow 
usually one year is sufficient to restore it. 

The fallow land may be kept bare throughout the whole 
period or it may be cropped, but the crop should not be re- 
moved from the land. A " catch crop " is sometimes taken 
in the autumn, and is either eaten off or ploughed in green. 
More frequently a root crop is raised and eaten off. This 
does not interfere with the operations of cleaning and cul- 
tivating the land. In either case the crop adds nothing 
to the land. If it be consumed on the spot, very little plant 
food is actually removed from the soil. Assuming that 
all, or nearly all, the plant foods are returned to the soil, 
a cropped fallow is better than a bare one, because apart 
from other advantages xwhich cannot be discussed here 
the crops take up the plant food that is rendered avail- 
able and preserve it from loss. If the fallow crops roots, 
etc. be removed from the land the benefit of the fallow, 
as a means of regenerating the supply of available plant 
food, is entirely lost, and must be compensated in other 
ways. In that case the only advantage is the opportunity 
afforded of cleaning the land. 

Laying down Land in Grass. This also is a very ancient 
device for the restoration of partly exhausted soils and 
the maintenance of fertility. It generally forms a regular 
part of the rotation or system of farming, and is often the 
subject of special stipulations in leases. The total annual 



FEETILITY 167 

produce of a grass field contains a large amount of plant 
food, and, if it were regularly cut and carried off, the 
fertility of the soil would not be restored but reduced. 
When the field is grazed the produce is all consumed, but 
only a comparatively small proportion of the plant foods 
is permanently retained by the animals. Generally, 1 in 
round numbers, about three-fourths of the phosphoric acid, 
nine-tenths of the nitrogen, and practically all the potash 
are restored to the soil in the droppings. The quantity of 
available plant food permanently abstracted from the soil 
is thus greatly reduced, and, when the land is ploughed 
up after several years lay, it will be found that they have 
accumulated and that the productiveness has considerably 
increased. If the grass is cut in alternate years, or more 
or less frequently, the benefit to the land will be corre- 
spondingly reduced. 

Restitution. It has been shown above that a cropped 
fallow and laying down the land in grass are advantageous 
as a means of increasing the supply of available plant food 
in the soil only when the crops are consumed on the land, 
and that the benefit consists essentially in restoring the 
ingredients of the crops to the soil. It frequently happens, 
however, that it is more convenient to remove the crops 
and consume them in the stables and cowhouses of 
the farm. If the droppings of the animals are collected 
and carried back to the land again, the result is much the 
same as before, except that a certain amount of loss which 
may be minimised by careful management is incidental 
to the process. All the ingredients of the crops, save the 
small proportion retained by the animals and what is lost, 
may be thus restored to the soil. Boots and hay are gene- 

1 The amounts retained depend on the kind, age, and condition of 
the animals. Of the total nitrogen in the food consumed, fattening 
sheep and oxen retain about 5 per cent., pigs about 15 per cent., and 
milking cows about 25 per cent. 



168 SOILS AND MANUEES 

rally disposed of in this way. If they are sold off the farm 
the whole of the ingredients are lost, and the soil is im- 
poverished to that extent. In the case of corn crops the 
grain alone is usually sold ; the straw is retained, and either 
fed to the stock or used as litter, and so ultimately returned 
to the soil. 

The introduction of animals enlarges the cycle of changes 
thus : 



Crops - > Stock 




Soil < - Muck 



Starting from the soil the arrows show the course of the 
plant food. It is absorbed first by the crops and then by 
the stock, or the plants may undergo decomposition 
directly, i.e., without passing through the bodies of 
animals. Generally some goes in both ways, but, in any 
case, it is all finally returned to the soil. A certain 
amount of plant food may be withdrawn from the cycle 
in the form both of vegetable and of animal products, and 
the fertility of the soil will not be diminished unless the 
amount withdrawn exceeds what becomes available in the 
same time. 

Quantity of Available Plant Food required by Crops. 
The following table shows the average weight per acre 
of the ordinary farm crops and the quantities of nitro- 
gen, phosphoric acid, potash, soda, lime, magnesia, and 
silica contained in them. 



FERTILITY 169 

WEIGHT AND AVERAGE COMPOSITION OF FARM CROPS, IN LBS. PER ACRE. 






Nitrogen 


Phosphori 
Acid. 


Potash. 


Soda. 


Lime. 


Magnesia 


Silica. 


Wheat, grain, 30 bush. 
,, straw 

Total crop . 


Ibs. 
33 
15 


Ibs. 
14-2 
6-9 


Ibs. 
9-3 
19-5 


Ibs. 
0-6 
2-0 


Ibs. 
1-0 

8-2 


Ibs. 
3-6 
3-5 


Ibs. 
0-6 
96-3 


48 


21-1 


28-8 


2-6 


9-2 


7-1 


96-9 


Barley, grain, 40 bush. 

,, straw 

Total crop . 


35 
13 


16-0 
4-7 


9-8 
25-9 


1-1 
3-9 


1-2 

8-0 


4-0 
2-9 


11-8 

56-8 


48 


20-7 


35-7 


5-0 


9-2 


6-9 


68-6 


Oats, grain, 45 bush. . 
,, straw . 

Total crop . 


38 
17 


13-0 
6-4 


9-1 
37-0 


0-8 
4-6 


1-8 
9-8 


3-6 
5-1 


19-9 
65-4 


55 


19-4 


46-1 


5-4 


11-6 


8-7 


85-3 


Meadow hay, 1| tons . 


49 


12-3 


50-9 


9-2 


32-1 


14-4 


56-9 


Bed clover hay, 2 tons 


102 


24-9 


83-4 


5-1 


90-1 


28-2 


7-0 

0-4 
6-9 


Beans, grain, 30 bush. 

,, straw 

Total crop . 


77 
29 


22-8 
6-3 


24-3 

42-8 


0-6 
1-7 


2-9 
26-3 


4-2 

5-7 


106 


29-1 


67-1 


2-3 


29-2 


9-9 


7-3 


Turnips, root, 17 tons . 
leaf 


63 
49 


22-4 
10-7 


108-6 
40-2 


17-0 

7-5 


25-5 
48-5 


5-7 
3-8 


2-6 
5-1 


Total crop . 


112 


33-1 


148-8 


24-5 


74-0 


9-5 


7-7 


Swedes, root, 14 tons . 
leaf 


70 

28 


16-9 

4-8 


63-3 
16-4 


22-8 
9-2 


19-7 
22-7 


6-8 
2-4 


3-1 
3-6 


Total crop . 


98 


21-7 


79-7 


32-0 


42-4 


9-2 


6-7 



170 



SOILS AND MANUEES 



WEIGHT AND AVERAGE COMPOSITION OF FARM CROPS, IN LBS. PER ACRE 
continued. 






Nitrogen. 


Phosphoric 
Acid. 


Potash. 


Soda. 


Lime. 


Magnesia. 


Silica. 


Mangolds, root, 22 tons 
leaf . 


Ibs. 

87 
51 


Ibs. 
36-4 
16-5 


Ibs. 

222-8 
77-9 


Ibs. 
69-4 
49-3 


Ibs. 
15-9 
27-0 


Ibs. 
18-3 
24-2 


Ibs. 
8-7 
9-2 


Total crop . 


138 


52-9 


300-7 


118-7 


42-9 


42-5 


17-9 


Potatoes, tubers, 6 tons 


47 


21-5 


76-5 


3-8 


3-4 


6-3 


2-6 


Beech, wood 
,, leaf litter. 


10 
39 


1-5 

9-3 


4-2 

8-8 


0-8 
1-6 


12-9 
73-1 


3-4 
10-9 


2-2 
53-9 


Total produce . 


49 


10-8 


13-0 


2-4 


86-0 


14-3 


56-1 



The total quantity of nitrogen, phosphoric acid and 
potash contained in the crops of an ordinary four- course 
rotation may be estimated approximately as follows : 



- 


Nitrogen. 


Phosphoric 
Acid. 


Potash. 


Boot crops (turnips, swedes and mangels, 








average, including leaf) 


116 


36 


176 


Corn crops (wheat, barley and oats, 








average, including straw) 


50 


21 


37 


Seeds (clover and meadow hay) 


75 


18 


67 


Corn crops, as before .... 


50 


21 


37 


Total in four years 


291 


96 


317 


Average, yearly 


73 


24 


79 



These figures do not include the ingredients contained 
in the stubble and root portions of the hay and corn crops 



FEETILITY 171 

which are left in the soil, and which must be taken into 
account in estimating the quantities of plant food annu- 
ally required by the crops. It appears, therefore, that, 
in order to provide for the requirements of the crops, the 
soil must be able to supply, annually, in round numbers, 
about 100 Ibs. each of nitrogen and potash and 30 Ibs. 
of phosphoric acid in an available state. Taking the mass 
of the soil, as before, at 2,500,000 Ibs. per acre, these 
quantities are equivalent to 0*004 per cent, of nitrogen 
and of potash, and 0'0012 per cent, of phosphoric acid. 
Of course, in order to provide the quantities required by the 
crops, the soil must contain a considerable excess probably 
many times as much as is actually abstracted by the plants. 

Loss of Plant Food by the Sale of Produce. The figures 
given above show the quantities of plant foods required to 
produce the crops. If they were removed from the land 
and nothing put back, the annual loss would be at the 
average rate of about 73 Ibs. of nitrogen, 24 Ibs. of phos- 
phoric acid, and 79 Ibs. of potash. In that case the pro- 
cess of exhaustion would be very quickly accomplished. 
It is, however, impossible to abstract the plant foods from 
the soil at that rate because, if nothing were put back, it 
would be impossible to obtain crops of the magnitude 
assumed. 

If only the grain of the corn crops and the meat pro- 
duced by feeding the roots and hay were sold the loss would 
be greatly reduced. It may be estimated approximately 
as shown in the table on p. 172. 

The loss is thus reduced to less than half the quantity 
of phosphoric acid and a third of the nitrogen containe'd 
in the crops. The loss of potash is very small. The loss 
of plant foods would, of course, be much greater if any 
portion of the hay or roots were sold, or if milk were 
sold instead of meat. If only butter were sold instead 
of milk the loss would be less, and so on. 



172 



SOILS AND MANURES 






Per Acre. 


Nitrogen. 


Phosphoric 
Acid. 


Potash. 


Grain of two corn crops .... 
By feeding roots ..... 
By feeding hay ..... 


Lbs. 

70 
11 

7 


Lbs. 

28 
9 
4 


Lbs. 

19 
2 

1 


Total in four years . . v . 


88 


41 


22 


Average, yearly 


22 


10 


5 



Effect of the Loss of Plant Foods on the Fertility of the 
Soil. It is obviously of great importance to determine 
whether this practice is theoretically justified, i.e., to com- 
pare the rate of loss of plant food by the sale of produce, 
with the rate at which it is converted into an available 
state. If the former exceed the latter, the fertility must 
depreciate. Unfortunately it is not known at what rate 
plant food is converted into an available state in any soil 
under any conditions. It cannot be exactly determined 
because if for no other reason the term available is not 
definable. Substances that are assimilated by one kind of 
crop cannot be assimilated by others. Some idea, may, how- 
ever, be formed from the yield of the unmanured plots at 
Kothamsted. Crops have been grown on these plots and 
removed year after year for more than fifty years. During 
that time nothing has been put back and nothing has been 
added, but the land has been carefully cleaned and cul- 
tivated all the time. The records show that the yield from 
these plots is now much smaller than it was when the 
experiments were begun. If it be assumed that the surplus 
of available plant foods has been exhausted, the quantities 
contained in the produce of these plots should correspond, 



FERTILITY 



173 



in some degree, to the rate at which the plant food is con- 
verted from the non-available to the available state. 

The yield of wheat, barley, hay and roots (mangolds) 
in 1907, and the estimated quantities of nitrogen, phos- 
phoric acid and potash contained in the same are given 
in the following table: 

YIELD OF CROPS FROM THE CONTINUOUSLY UNMANURED PLOTS AT 
ROTHAMSTED AND THE QUANTITIES OF PLANT FOODS IN THE 

SAME. 

PER ACRE. 






Nitrogen. 


Phos- 
phoric 
Acid. 


Potash. 


Barley . j 


grain, 7'7bush. = 412 Ibs. 
straw, 6-8 cwt. = 762 


Lbs. 

7-0 
4-0 


Lbs. 

3-2 
1-5 


Lbs. 
9-8 
8-0 




Total .... 


11-0 


4-7 


17-8 


Wheat . | 


grain, 9'1 bush. = 552 Ibs. 
straw, 9-8 cwt. = 1,098 


9-9 
4-6 


4-3 
2-1 


2-8 
6-0 




Total .... 


14-5 


6-4 


8-8 


Mangolds . j 


root, 5-15 tons =11,536 Ibs. 
leaf, 1-06 ,, = 2,374 


20-4 
2-0 


8-5 
0-9 


52-2 
2-5 




Total .... 


22-4 


9-4 


54-7 


Meadow hay 


. 23-1 cwt. = 2,587 Ibs. 


37 


9-4 


39 


Highest 




37 


9-4 


55 


Lowest 


. 


11 


4-7 


8-8 


Average 


of four crops 


21 


7'5 


30 



174 SOILS AND MANUEES 

As the productiveness in respect of some of the crops 
appears to be still diminishing, it is probable that the 
surplus store of available plant foods is not yet entirely 
exhausted, and that the estimate is too high. In other 
words, it is probable that plant food does not become 
available to these crops so rapidly as is indicated. In 
any case, owing to the difference in the assimilative 
capacities of the crops, it is impossible to arrive, by this 
method, at any very satisfactory conclusion in regard 
to the rate at which the plant foods become avail- 
able. It must be remembered also that when nitrogen 
alone is added larger amounts of phosphoric acid, etc., 
are absorbed by the crops. Still it is not without 
interest to notice that, at the lowest computation, the 
potash is more than sufficient, and, at the highest, the 
phosphoric acid is barely sufficient to make good the 
losses of these ingredients by the sale of grain and meat. 
Taking the average of the four crops, it will be seen that 
the nitrogen is just equal to, and the phosphoric acid 
is substantially less than, what is required. There are, 
of course, other sources of loss, particularly of nitrogen, 
incidental to the conduct of the operations, and which tend 
to reduce the supply of available plant foods. In a general 
way, therefore, these considerations confirm the conclu- 
sions arrived at by practical experience, viz., that the 
sale of grain and meat tends gradually to reduce the fer- 
tility of the soil, but the deficiency is small and, on a 
moderately rich soil, could probably be made good by occa- 
sional fallowing and pasturing. Land has been farmed 
for hundreds of years, and, in some districts, is still farmed 
on these or similar lines, i.e., grain and meat, or their 
equivalents, alone are sold ; the rest is put back, but 
nothing is given to the land except what is produced 
from it. 

Addition. The highest degree of productiveness cannot, 



FERTILITY 



175 



however, be attained, and, on the poorer soils, probably a 
profitable return could not be obtained, unless the materials 
lost by the sale of produce, and in other ways, were replaced 
from external sources. In short, it is generally necessary 
and advisable to add plant foods, as well as to restore a 
large proportion of what is taken out of the soil by the 
crops. Plant foods may be added to the soil directly, or 
they may be purchased in the form of food for animals and 
be afterwards added to the soil in the form of farmyard 
manure. There are thus two points at which materials 
may be introduced into the cycle to compensate for the 
losses by sale of vegetable and animal produce. The cycle 
remains as before, and the importation of extraneous 
matters may be indicated in the following manner : 



Soil 



Crops 




Muck * 



It is evident that if plant foods are purchased and added 
to the soil in the form of what are known as fertilisers, 
or artificial manures, larger crops can be produced on the 
same land ; a larger amount of vegetable and animal pro- 
duce may thus be sold, and there will be more to go back 
to the land. If, instead of raising larger crops by means 
of artificial manures, a quantity of food for animals be 
purchased, more stock can be kept; the animals retain, 
permanently, only a small proportion of the fertilising 
ingredients of the food ; the remainder goes back to the 
land, and larger crops may be produced. 

Essential Difference between Purchased Fertilisers and 
Farmyard Manure. There is thus a great fundamental 



176 SOILS AND MANUEES 

difference between purchased fertilisers and feeding stuffs 
on the one hand, and farmyard manure and food for 
animals grown on the farm on the other. The former is 
something added to the land, i.e., introduced into the cycle ; 
the latter is merely a means of restoring to the land what 
had been previously taken out of it, i.e., a stage in the 
cycle of changes which might, theoretically, go on for ever 
without gain or loss, except by the conversion of plant 
food into the available state in the soil and by the sale of 
produce. If the whole of the increase due to the use of 
purchased manures and feeding stuffs be sold, the fertility 
of the soil will not be increased. It is sometimes said that 
more cannot be got out of the soil than is put in. 
This is not so. The produce of the land may be equal to 
what is put in plus what becomes available in the same 
time. 

Conservation of Matter. The facts quoted above form 
an illustration of the law of conservation of matter. It 
asserts that " the total quantity of matter is not altered 
by the changes that it undergoes." It is one of the funda- 
mental laws of nature, and applies equally to physical, 
chemical and biological changes. Most farmers are more 
or less conscious of the operation of this law in regard to 
agriculture, but they sometimes fail to realise its full im- 
portance. The weight of the crops is usually so much 
greater than that of the seed sown, as almost to make it 
appear as if something had been formed out of nothing. 
The truth is, of course, that the plants have merely trans- 
formed materials which they have collected from the air 
and from the soil. One hears occasionally of men keeping 
more stock than they can properly feed, in order to make 
manure. It is quite certain that nothing comes out of 
the animals that has not previously gone in. The animals 
transform the constituents of the food into meat, milk, 
wool, etc., but they do not produce these things out of 



FEETILITY 177 

nothing. If the animals are fed on nothing but straw 
the dung will contain nothing but what the straw contains. 
If the food is enriched by the addition of oil cake, etc., 
the dung will be enriched by the constituents of the oil 
cake. The quantity of plant food in the soil may be in- 
creased by addition either from natural sources or in 
other ways. It may be diminished by abstraction by crops 
or by accidental losses. The quantity will remain con- 
stant only when the additions are exactly equal to the 
abstractions. A certain minimum quantity of plant food 
is necessary for the production of a maximum crop; 
if less than this minimum be present the crop will be 
correspondingly decreased. 



S.M. 



CHAPTEE VII 

THE PRINCIPLES OF MANURING 

Definition of Manure. The word manure may be, per- 
haps, capable of definition in more than one way accord- 
ing to the point of view from which the subject is re- 
garded. It will, however, be used here to include all and 
only such substances as contain appreciable quantities 
of plant food in a condition suitable for assimilation 
by plants, or which readily change into such a condition 
and are directly incorporated with the soil. 

This definition, of course, includes farmyard manure 
the waste animal and vegetable matter which is restored 
to the soil to which the name manure has sometimes 
been exclusively applied. It also includes seaweed, town 
refuse and other animal and vegetable matters obtained 
from outside the farm, as well as mineral substances, 
and bye products from gasworks, ironworks, boneworks, 
etc. It does not, however, include oil cakes and other 
vegetable matter intended to be consumed by animals, 
though the farmyard manure may be subsequently en- 
riched in this way. It sometimes happens, however, 
that these products are damaged, become unfit for use as 
animal foods, and are directly incorporated with the soil ; 
in that case they would be regarded as manures. 

Lime might be considered as coming under the 
definition inasmuch as calcium compounds are essential 
constituents of plants. This, however, is quite a sub- 
ordinate function of " lime," properly so-called, and it 
is not generally added to the soil for this purpose, but 



THE PEINCIPLES OF MANUKING 179 

for a variety of other reasons which have been previously 
mentioned. It is unnecessary, therefore, to deal with the 
subject again under the head of manures. 

It has also been mentioned that certain manures may 
affect the physical and biological properties of the soil 
as well as the chemical composition. These effects must 
be taken into consideration in connection with the sub- 
stances which produce them, but, as a rule, they are of 
secondary, if sometimes of considerable importance. 
According to the definition given above, manures are to 
be regarded chiefly from the chemical point of view, i.e., 
as substances containing plant foods to be added or 
restored to the soil. 

The Constituents of Manures. It is obvious that a 
mixed product like farmyard manure, consisting largely 
of vegetable matter, must contain all the constituents 
of plants, and, if applied in sufficient quantity, should 
be able to satisfy all their requirements. But it is not 
generally necessary to supply all the constituents of the 
crops in the form of manure. Of the elements that are 
essential for the growth of plants, carbon, hydrogen and 
oxygen can be dismissed at once. The first is obtained 
from the air, and so far as the nutrition of the crops is 
concerned, it apparently makes no difference whether the 
soil contains much or little or, indeed, any carbon com- 
pounds at all. The presence of carbonaceous matter in 
the soil may make a considerable difference in other 
ways. Hydrogen and oxygen are derived mainly from 
water, and need not therefore be further discussed in 
this connection. 

Sulphur, iron and magnesia are usually present in 
the soil in quantities far in excess of those required 
by plants. The sulphates brought down to the soil in 
the rain water contain more than enough sulphur to 
make good all the loss of that constituent by removal of 

N2 



180 SOILS AND MANUEES 

crops. The quantity of iron contained in ordinary crops 
is infinitesimal and it is hardly conceivable that any 
natural soil should be deficient in that respect. Cases 
in which deficiency of iron has been suspected from the 
appearance of the crops have come under the writer's 
notice, but in none of them was it confirmed by examina- 
tion of the soil. The application of compounds of iron 
to the soil has occasionally produced a good effect on the 
growth of potatoes and some other crops, but it is difficult 
to believe that the result was due to the direct action of 
the iron as a plant food. 

Soils may be occasionally deficient in magnesia, and 
cases are on record in which the application of mag- 
nesium compounds is said to have produced a good effect. 
Such cases are, however, extremely rare. Magnesium 
compounds are usually fairly plentiful in the soil, and 
it will be seen on reference to the table (p. 169), that, of 
the ordinary farm crops, red clover and mangolds alone 
contain any considerable quantity. 

Apart from lime and the elements deemed to be non- 
essential, only nitrogen, phosphoric acid and potash re- 
main to be considered. Soils are usually more or less 
deficient in these constituents. Considerable quantities 
of nitrogen and phosphoric acid are lost by the sale of 
produce, and a certain amount of nitrogen is unavoidably 
lost in other ways. These therefore are the most impor- 
tant constituents of manures. It is chiefly these sub- 
stances that must be restored to and added to the soil. 
In estimating the value of manures as sources of plant 
foods, only the nitrogen, phosphoric acid and potash are 
considered. They are sometimes referred to as the 
manurial elements or the fertilising ingredients of 
manures. 

Functions of Manures. The application of manures 
to the soil may be merely an act of restitution, i.e., a 



THE PRINCIPLES OF MANURING 181 

process of replacing plant food previously abstracted 
from the soil and which cannot be removed without de- 
preciating the fertility ; or it may be an addition of plant 
food made for the purpose of increasing the productiveness 
of the soil beyond its natural capacity. The functions 
of manures are therefore twofold to maintain fertility 
and to increase it. 

Eestoration is effected chiefly by farmyard manure, 
and that is its principal function. So long as the con- 
stituents of farmyard manure are all derived from the 
soil it cannot possibly increase fertility; at best, it 
can only maintain it, unless the sale of produce is 
restricted to less than the equivalent of the quantity of 
plant food that is converted into an available state during 
the period of production ; if the sale of produce were so 
restricted it would be difficult to obtain a profitable 
return. The fertility of the land may, however, be in- 
creased by the use of farmyard manure if it is wholly or 
partially derived from external sources. For example, 
if it be not produced from the soil but purchased from 
town stables, etc., as is usually the case in gardens, or 
if it contain fertilising ingredients derived from oil 
cake or other purchased foods consumed by animals. 

As it is generally impossible to purchase farmyard 
manure, addition can, as a rule, only be made in the form 
of the so-called artificial manures. To increase fertility 
not to maintain it is therefore the principal function 
of these substances. It is obvious, of course, that if, 
for any reason, the farmyard manure were withheld, an 
equal quantity of plant food could be restored to the 
soil in the form of artificial manure. Whether it would 
serve the purpose equally well is a matter for further 
consideration. 

In general, the plant foods abstracted from the soil in 
the course of the rotation are restored in the farmyard 



182 SOILS AND MANUEES 

manure, and artificial manures are supplied to the differ- 
ent crops to increase production. 

Adaptation of Manures to Circumstances. Artificial 
manures may contain all the fertilising ingredients 
nitrogen, phosphoric acid and potash or only one or 
two. They are commonly referred to as nitrogenous, 
phosphatic or potash manures according to the nature 
of the predominant constituent. There are several kinds 
of each, differing more or less in their properties, and 
they must be used with discretion in order to obtain 
satisfactory results. Comparisons are sometimes drawn 
between the effects of artificial manures and farmyard 
manure, but no just comparison can be made except 
when one is used as a substitute for the other. If it be 
proposed to employ artificial manure as a substitute 
for farmyard manure, i.e., for purposes of restitution, 
all three constituents must be supplied. They may be 
derived from one or several sources, and may be applied 
on one or several occasions, but the total quantity of 
each should be equivalent to the quantity contained in 
the dressing of farmyard manure to which the artificial 
manure corresponds. Even if chemically equivalent, 
the artificial manure would not, of course, produce the 
physical and biological effects that farmyard manure 
does. When artificial manures are employed as addi- 
tive substances, i.e., to increase production beyond the 
natural capacity of the soil, they must be properly 
adapted to the requirements of the soil and of the crop 
to which they are applied. 

Requirements of the Soil. In considering the require- 
ments of the soil, both the physical properties and the 
chemical composition should be taken into account. 
Manures in which the plant foods only become available 
as a result of oxidation, act better on light, open soils 
than on those of closer texture ; those which are very 



THE PEINCIPLES OP MANUBING 183 

soluble are less liable to loss by drainage on stiff land 
than on open sandy soil. 

The chemical composition of soils is sometimes abnor- 
mal. There may be a deficiency or there may be an 
excess of some particular ingredient, or the soil may be 
more or less deficient in all the essential elements of 
plant food. Clay soils derived from granite are usually 
well supplied with potash. Sandy soils and humous 
soils are very often deficient in that ingredient. The 
proportion of nitrogen in humous soils is generally 
large compared with that of the other ingredients ; owing 
to the rapid oxidation and free drainage, sandy soils are 
often conspicuously deficient in nitrogen. The growth 
of leguminous crops tends to enrich the soil in nitrogen 
and to exhaust the potash and phosphates. Other crops, 
especially roots and potatoes, tend to exhaust the potash 
and phosphates without increasing the nitrogen. The 
manures applied to previous crops generally prevent 
exhaustion and leave a certain residue for those which 
follow. 

It is obviously superfluous to add those ingredients 
which are already present in excess, and it is essential 
to add those of which there is a deficiency. This has 
been formulated in a statement sometimes called the law 
of maxima and minima. It asserts that the crop is 
governed by the constituent which is present in minimum 
quantity. According to this view, each kind of crop 
requires a certain minimum quantity of each of the 
essential constituents, and a deficiency of one is not in 
any way compensated by excess of any or all of the 
others. For example, assuming that a crop requires, say, 
50 Ibs. of nitrogen, 30 Ibs. of potash, and 20 Ibs. of 
phosphoric acid, a full crop could only be obtained if 
the soil were able to furnish these quantities ; if it 
could only provide half the quantity of one of them 



184 SOILS AND MANURES 

say phosphoric acid only half a crop could be pro- 
duced, notwithstanding the presence of enough nitrogen 
and potash for a full crop. The crop would not be 
increased by adding more nitrogen or potash or all the 
other constituents, but only by adding that which is 
deficient phosphoric acid. 

This is doubtless true in the case of an artificial 
soil in which all the constituents are present in an avail- 
able state, but it is not strictly accurate in regard to 
ordinary soils which contain plant foods in many differ- 
ent degrees of solubility. It is found that when a soil 
has become, by, exhaustion, deficient in both nitrogen 
and phosphates, the addition of nitrogenous manures 
alone will increase the yield and, further, that by con- 
tinuing the applications of nitrogenous manures a larger 
yield can be obtained for many years in succession. It 
would appear as if the nitrogenous manures so stimu- 
lated the growth of the plants as to enable them to 
assimilate compounds .which were otherwise not avail- 
able. The application of phosphatic manures alone, 
in such a case, would also increase the crops. In neither 
case, however, would a maximum crop be obtained. If 
a soil be deficient in both nitrogen and phosphates a 
maximum crop can only be obtained when phosphatic 
and nitrogenous manures are applied together. The in- 
crease produced by the joint action of the manures will 
generally be much greater than that produced by the 
two manures acting separately. This is prominently 
brought out by some of the experiments at Eothamsted, 
and may be illustrated by the table on p. 18,5 taken from 
the records. 

Too Rich Soils. In general manures may be said to 
be adapted to the requirements of the soil when they can 
supply any ingredient of which there is a deficiency. 
But an undue excess of any of the constituents may be 



THE PBINCIPLES OF MANUBING 
PER ACRE. 



185 





Hay. 


Barley. 




Crop. 


Increase. 


Crop. 


Increase. 




Cwts. 


Cwts. 


Bushels. 


Bushels. 


Unmamired .... 


23-1 




7'7 




Nitrogenous manure . 


28-3 


5-1 


19-9 


12-2 


Phosphatic manure 


27-0 


3-9 


13-0 


5-3 


Nitrogenous and phosphatic 










manure ..... 


42-5 


19-4 


28-4 


20-7 



nearly as bad as a deficiency. When farmers speak of a 
soil as being "too rich," they generally mean that it 
contains too much nitrogen. Under these conditions 
corn crops tend to run more to straw than grain and to 
lie down, grass becomes rank and coarse, and root crops 
ripen before they are fully developed. These effects can 
be, at least partially, mitigated by increasing the pro- 
portions of the other constituents, especially phosphates 
and potash, even though the soil may not be judged 
by ordinary standards deficient in these ingredients. 
The requirements of the soil must therefore be gauged 
not wholly by the absolute quantities of the constituents 
but, to some extent, also by the relative quantities. 

DETERMINATION OF THE MANURIAL REQUIREMENTS OF 
A SOIL. 

Eight-plot Test. Farmers who have cultivated a piece of 
land for some years can generally tell what are its prin- 
cipal manurial requirements and what is the most suitable 
form of manure to apply. If there be any room for doubt 
it should be made the subject cf special investigation, and 
it is better to proceed on definite systematic lines, so as 
to settle the matter in a single season, than to try 



186 



SOILS AND MANUEES 



casually for several years. The object is, generally, to 
try the effects of nitrogenous, phosphatic and potash 
manures and the different combinations of the same. By 




10 x 20 
20*40 



(nearly f 



Phosphate Potash Nitrogen 

FIG. 25. 



comparing the results with each other and with that 
obtained on an unmanured piece, it is generally possible 
to determine what is required. This experiment is com- 
monly known as the " eight-plot test," because that is 



THE PEINCIPLES OF MANURING 187 

the number of plots required to carry it out in its en- 
tirety. The plots may be of any size and may be 
arranged in any order that is convenient. It is ad- 
visable, however, that they should be not too large. 
When the plots are small it is easier to compare the 
effects of the manures, a smaller area is involved and 
correspondingly smaller quantities of the manures are 
required. The following arrangement is a compact one 
and gives, perhaps, less trouble than any other in apply- 
ing the manures ; the total area of land involved is 
approximately a third of an acre and the quantities of 
manure required are 1 -cwt. of superphosphate, or basic 
slag, ^ cwt. of kainite, and J cwt. of nitrate of soda, or 
sulphate of ammonia. The phosphatic manure (super 
or slag) is applied to a piece of land 40 yards long by 
20 yards broad, and the kainite to a similar piece over- 
lapping half the breadth (10 yards) of the first through- 
out the length. The nitrogenous manure (nitrate of 
soda or sulphate of ammonia) is applied as a cross 
dressing to the lower half, and extended 10 yards beyond 
the margin of the manured piece. These instructions will 
be more readily followed by comparing them with the plan. 

The phosphate is applied to the piece PI, P 2 , PS, . P , 
shaded with vertical lines. The kainite is applied to 
the piece K b K 2 , K 3 , K 4 , shaded with dots. The middle 
portion K b K 2 , P 3 , P 4 , thus gets both phosphate and 
potash ; of the other two portions, one gets phosphate 
alone and the other gets kainite alone. The nitrogenous 
manure is applied to the piece NI, N 2 , N 3 , N 4 , shaded with 
horizontal lines. 

The result is altogether eight plots manured as follows : 

Upper half (P a K 2 ), phosphate alone. 

,, (K 2 P 3 ), phosphate and kainite (no nitrogen). 

,, (P 3 K 3 ), kainite alone. 

,, (K 3 0), no manure. 



188 SOILS AND MANUBES 

Lower half (Pi KI), phosphate and nitrogen (no potash). 

,, (Ki P 4 ), phosphate, kainite, and nitrogen (complete 

manure). 

,, (P 4 K 4 ), kainite and nitrogen (no phosphate). 

,, (K 4 N 3 ), nitrogen alone. 

It will be seen that there are three plots which re- 
ceive only one kind of manure phosphates alone, potash 
alone, and nitrogen alone and three from which these 
substances are respectively withheld, the other two being 
added. The former should be compared with the un- 
manured plot, the latter with the complete manured 
plot. The special manurial requirements of the soil 
may thus be judged, both from the effects of adding a 
particular kind of manure and from the effects of with- 
holding it. 

The results can best be ascertained by gathering the 
produce of each plot and weighing it, but if they are 
not to be recorded, simple inspection will generally be 
sufficient. If the effects of the manures are not apparent 
to the eye it may be either because the soil does not 
require manure at all or because it requires some other 
treatment as well. 

When the experiment is conducted on purely qualita- 
tive lines, i.e., when the results are to be ascertained 
by simple inspection, it is not necessary that the measure- 
ments should be very exact. They might be dispensed 
with altogether but for the necessity of adjusting the 
quantities of the manures, at least approximately, to the 
area of land. In the arrangement described above, the 
plots are 10 yards by 20 yards, which is very nearly ^jth 
part of an acre, and the quantities of manure given are 
at the rate of 6 cwt. of phosphate, 3 cwt. of kainite and 
1J cwt. of nitrate of soda per acre. 

If the results are to be recorded, the experiment must 
be quantitative and should be carried out with the exact- 



190 SOILS AND MANURES 

ness of a scientific investigation. The scheme of " joint 
experiments " published by the Board of Agriculture 
includes one for the determination of the manurial re- 
quirements of the soil. It does not differ in principle 
from that described above, but is drawn up with a view 
to accuracy rather than simplicity. The results will 
be of much greater value for purposes of comparison 
with those of other workers if the instructions given in 
the scheme of joint experiments are followed in detail. 

The eight-plot test generally affords a fairly reliable 
indication of the special manurial requirements of the 
soil, but it is by no means infallible. The results depend, 
to Borne extent, upon the crop to which the experiment is 
applied, and the requirements of the crop must, therefore, 
be taken into consideration in drawing inferences. Apart 
from this, the experiment sometimes fails to return any 
definite and satisfactory answer to the question. This may 
be due, not to any fault in the test, but to the fact that, in 
certain circumstances, it is not applicable. It does not 
take into account certain other conditions which have a 
considerable influence on productiveness. 

The Wire-basket Method. K quick method of testing 
the manurial requirements of soils has recently been 
devised by the American Bureau. 1 It consists in growing 
the plants in small 'wire pots as shown in the illustration 2 
(Fig. 26) containing the soil to be tested, to which differ- 
ent kinds and quantities of fertilising ingredients are 
added. The seeds are sprouted before being planted, in 
order to secure uniformity and to accelerate the operation. 
The baskets are coated with an- impervious layer of 
paraffin, and when the plants have reached a height of 
about two inches, are sealed with discs of paper dipped 

1 Bulletin 18, March, 1905. 

2 Bui. 2, New Zealand Dept. of Agriculture. 



THE PRINCIPLES OF MANURING 191 

in the same material. The discs are perforated with 
holes 'to allow the plants to grow and through which 
water may be added. The pots are weighed every two 
or three days in order to measure the loss of water by 
transpiration. At the end of three or four weeks the 
plants are cut and weighed and the amount of dry 
matter in them is estimated. Both sets of data are con- 
sidered in estimating the effects of the fertilising in- 
gredients, and the results correspond closely. There are 
many possible sources of error in the conduct of the 
operation and great care is required to secure com- 
parable results. In any case the method cannot be 
considered as reliable as field trials. Its chief advan- 
tage lies in the comparatively short time in which it can 
be carried out. 

The illustration 1 ,] (Fig. 27) shows the results obtained 
in two tests applied to the same soil ; the manures were 
the same, but different kinds of plants were used that in 
the upper figure being rape, and that in the lower, clover. 

Chemical Analysis. The manurial requirements of the 
soil may also be determined by chemical analysis. For 
this purpose the investigation may be confined to the esti- 
mation of the total nitrogen and available phosphoric acid 
and potash. It is quicker than field trials and the results 
are not obscured by the peculiarities of a growing crop. 
It may be easily extended so as to tal^e into account other 
considerations, and it has sometimes succeeded when the 
eight-plot test has failed. It is more expensive than the 
11 practical " test and is generally considered not so reli- 
able. So long as a suspicion of this kind remains, the 
conclusions arrived at by chemical analysis must be subject 
to confirmation by experiments in the field. The two 
methods may therefore be regarded as supplementary. 

1 Bui. 2, New Zealand Dept. of Agriculture. 



192 



SOILS AND MANUEES 
REQUIREMENTS OF THE CROPS. 



Chemical Composition. The chemical composition of 
the crops does not afford a very clear indication of their 
manurial requirements, because some of them can obtain 



FIG. 28. 

a larger proportion of the fertilising ingredients directly 
from the soil and are less dependent upon manures 
than others. Any deductions drawn from the chemical 
composition of the crops cannot, therefore, be regarded 
as conclusive unless all the other circumstances con- 
nected with their growth have also been taken into 



THE PEINCIPLES OF MANURING 



193 



consideration. Still it is true, in general, that those 
which contain the largest quantities of fertilising in- 
gredients require most manure, and that any crop which 
contains an exceptionally large proportion of any par- 
ticular constituent usually gives a larger return for 
those kinds of manure which supply that constituent. 

Division of Crops into Groups. The average com- 
position of the ordinary crops has been given in Ibs. per 
acre (p. 169). It will be seen on reference to the table, 
that crops which belong to the same class resemble each 
other more or less closely in this respect and may be 
arranged in groups accordingly. It is shown, perhaps 
more clearly in the diagram (Fig. 28), that the grain crops 
and grass all contain similar quantities of nitrogen, phos- 
phoric acid and potash ; clover, peas and beans each con , 
tain about the same quantities of these ingredients ; so 
also do turnips and swedes ; potatoes and mangolds cannot 
be referred to any of these groups nor can they be classed 
together. 

In round numbers the quantities of nitrogen, phos- 
phoric acid and potash in the crops of the several groups 
are as follows : 

PER ACRE. 






Nitrogen. 


Phosphoric 
Acid. 


Potash. 


Grain crops and grass . 


Lbs. 
50 


Lbs. 
20 


Lbs. 

40 


Peas, beans, and clover 


100 


26 


70 


Turnips and swedes 


100 


26 


110 


Potatoes ... 


50 


22 


80 


Mangolds ... 


180 


55 


300 



It will be seen that all the crops contain considerably 
larger quantities of nitrogen and potash than of phos- 
phoric acid, but it does not follow that they stand less 

S.M. o 



194 SOILS AND MANUEES 

in need of phosphatic than of nitrogenous or potash 
manures. From the fact that the graminaceous crops 
contain less, and mangolds more, phosphoric acid than the 
others which are all much alike in this respect it may 
be supposed that the former require less, and the latter 
more, phosphatic manure than the other crops. The 
same is true, in part, also of the other constituents. 
It is also noticeable that in the graminaceous and legu- 
minous crops the proportion of nitrogen exceeds that 
of the potash and in the other crops the reverse is the 
case. In potatoes and mangolds the excess of potash is 
very conspicuous. The proportion of nitrogen to potash 
is about the same in the two crops, but the quantities of 
all the ingredients are much greater in the latter. 

The most important inferences, relating to the manurial 
requirements of the crops, that can be drawn from their 
chemical composition, are as follows : 

Graminaceous crops contain smaller quantities of all 
the fertilising ingredients than any of the other crops ; 
nitrogen is the largest of the three constituents. 

The leguminous crops contain larger quantities of all 
the ingredients but especially of nitrogen and potash. 

The root crops not including mangolds contain about 
the same quantity of nitrogen as the leguminous crops 
but much more potash. 

Potatoes contain less nitrogen bat nearly as much 
potash as the turnips and swedes. 

Mangolds contain much the largest quantities of all 
the fertilising ingredients. 

Assimilative Capacity. The various kinds of crops 
exhibit great differences in their powers of assimilating 
the plant foods in the soil. Those which have greater 
assimilative capacity are less dependent on manures. 
Of the ordinary crops, cereals and grass have the greatest 
power of assimilation, and root crops and potatoes 



THE PEINCIPLES OF MANUEING 195 

the least. The large proportion of silica in the former 
is regarded as a proof that silicates are available to 
these crops that are not available to the latter. Whether 
this is the true and whole meaning of the fact or not, 
it is a matter of common knowledge that grass and 
grain crops which are only a kind of grass after all 
can be grown on land that would not yield a crop of 
roots at all without manure. 

Cereals and Grass. The graminaceous crops require 
less manure than the others both because they require 
smaller quantities of the fertilising ingredients, and be- 
cause they are better able to ass'imilate them directly 
from the soil. If, on a soil of average good quality, 
the fertility is maintained at its normal level by the 
restoration of the plant foods in the farmyard manure 
at the end of each rotation, very good crops of grain 
and grass can as a rule be raised without the use 
of any special manure. The amount of produce can 
however, generally be increased by the application of a 
moderate top dressing of nitrogenous manure. Nitrate 
of soda appears to be the most suitable form. The bene- 
ficial effects of nitrogenous manures on these crops are 
probably due to the fact that the most active growth 
takes place in the early spring, i.e., just after the avail- 
able compounds of nitrogen nitrates have been ex- 
hausted by the winter drainage and before they have 
been replaced Jby the process of nitrification. If the 
fertility is not properly maintained by farmyard manure, 
phosphatic as well as nitrogenous manures should be 
applied to these crops. 

Root Crops. The root crops require very large quan- 
tities of plant food and their assimilative capacity is so 
inferior that, even on the naturally richest soil, full crops 
cannot be obtained unless they are well manured. It 
is partly for this reason that they are usually taken 

o 2 



196 SOILS AND MANURES 

first in the rotation, i.e., so that they may obtain the full 
benefit of the available plant foods restored to the land in 
the farmyard manure. In that case it is not usually 
necessary to give special nitrogenous or potash manures 
to turnips and swedes but a dressing of superphosphate 
generally proves a useful addition. The root crops are 
more favourably situated than the cereals for obtaining 
nitrogen directly from the soil, because their most 
active growth takes place later in the year and there is 
therefore a larger supply of available nitrogen produced 
by nitrification. This does not, however, apply to man- 
golds, for which nitrates are much more important than 
phosphates. Mangolds are deeper-rooted plants and 
better able to obtain phosphates directly from the soil 
than turnips and swedes. They are grown earlier in the 
year and the quantity of nitrogen contained in a crop 
is very large. 

Potatoes. The manurial requirements of potatoes re- 
semble those of turnips and swedes, but potatoes contain 
relatively more potash and give a better return for potash 
manures. Even a full dressing of farmyard manure will 
generally give a better crop when supplemented with a 
special potash manure. Though potatoes contain less nitro- 
gen than turnips and swedes, they are more dependent on 
manures for the supply of that element. Excessive 
quantities of nitrogenous manures," however, are some- 
times harmful, especially if the supply of potash be 
insufficient. Nitrogenous manures are superfluous when 
the customary quantities of farmyard manure are em- 
ployed. 

All these crops can, if necessary, be grown without 
farmyard manure if they are amply supplied with nitrogen, 
phosphoric acid and potash in the form of artificial 
manures. The farmyard manure must, however, be 
applied to the land at some time for purposes of resti- 



THE PEIKCIPLES OF MANUKING 197 

tution, and there is a general consensus of opinion that 
this can be done most suitably and economically when 
preparing the land for green crops. It sometimes hap- 
pens, however, that the supply of farmyard manure is 
too limited to allow of a dressing sufficient for all pur- 
poses, and the deficiency has to be made up by the use 
of artificial manures. In that case a certain quantity 
of all three constituents should be given, but the pre- 
dominant ingredient should be, for swedes and turnips, 
phosphates, for mangolds, nitrate of soda, and for pota- 
toes, potash manure. 

Leguminous Crops. The assimilative capacity of the 
leguminous crops is greater than that of the roots, but 
not so great as that of the cereals and grass. They con- 
tain larger quantities of the fertilising ingredients than 
the latter and stand more in need of special phosphatic 
and potash manures, but they do not require the direct 
application of the heavy dressings of farmyard manure 
that are necessary for the green crops. Like turnips 
and swedes, they contain a large quantity of nitrogen 
and, also like them, they do not require very large 
quantities of nitrogenous manures, but for a very differ- 
ent reason. When the soil is in a suitable condition and 
contains the necessary bacteria, the leguminous crops can 
utilise the free nitrogen of the air (p. 143), and are, to that 
extent, independent of nitrogenous compounds in the 
soil either naturally present or applied in the form of 
manures. They have, however, the power of absorbing 
nitrogenous compounds like other crops, and if the soil 
be not well supplied with available nitrogen, the appli- 
cation of a small quantity of nitrogenous manure often 
has a good effect. 

Garden Crops. The same general principles apply to 
the manuring of garden as of farm crops, but there are 
one or two points that deserve special consideration. 



198 SOILS AND MANURES 

With the exception of peas and beans, garden crops are 
not, as a rule, grown for the seed like corn crops. They 
may be conveniently divided into fruits, flowers and vege- 
tables. . They are not grown in definite, regular rotation, 
but are often taken from the same land several years 
in succession. Garden areas are usually much smaller 
than those of farms and can be much more thoroughly 
tilled by hand labour and more liberally manured. 

In many cases, enormous quantities of stable manure 
are used as a source of heat in order to secure early 
crops, and under these circumstances nothing else is 
required. The quantities of stable manure commonly 
employed, apart from the preparation of hot-beds, usually 
contain more than sufficient fertilising material to satisfy 
all the requirements of the plants. 

As a class, gardeners are opposed to the use of 
artificial manures, and rightly so if it is proposed to use 
them as a substitute for farmyard manure. For various 
reasons it is essential that garden soils should be well 
stocked with humus, and artificial manures do not gene- 
rally tend to increase that constituent but often have a 
contrary effect. Still, if there be any scarcity of stable 
manure, artificials may be used to supplement it and great 
economy can often be effected in this way. 

The kind and quantities of artificial manure to be 
employed must depend not only on the soil and crop, 
but perhaps even more largely upon the amount of 
farmyard manure employed. 

With the quantities of farmyard manure ordinarily 
used in gardens, the addition of a little superphosphate, 
is generally sufficient for .all purposes, provided the soil 
contains an adequate proportion of lime. It may be used 
at any rate up to about half a pound per square yard, 
but about half that quantity 4 oz. is generally more 
suitable. With smaller quantities of farmyard manure, 



THE PEINCIPLES OF MANUEING 



199 



nitrogenous manures and potash salts may also be used 
with advantage. 

Nearly all recipes for garden manures include a cer- 
tain amount of gypsum and ferrous sulphate (copperas), 
but for the reasons previously mentioned, these sub- 
stances cannot be expected to produce any great effect. 
Practical men appear to think that ferrous sulphate is 
really beneficial on calcareous soils, and even on ordinary 
soils, for certain crops, e.g., spinach. Gypsum, again, 
can only be useful on non-calcareous soils and its effects 
under any circumstances are more than doubtful. As 
a matter of fact some of the best informed French gar- 
deners have ceased to employ either of these substances. 

From the point of view of their manurial requirements, 
the vegetable crops may be grouped as follows : 



I. 


II. 


in. 


IV. 


Turnips. 


Artichokes. 


Cabbages. 


Peas and beans. 


Carrots. 


Potatoes. 


Spinach. 




Beetroot. 






Melons. 


Radishes. 


Onions. 


Parsley. 


Marrows. 


Parsnips. 


Leeks. 


Celery. 


Pumpkins. 




Shallots. 


Asparagus. 


Cucumbers. 






Lettuces. 


Tomatoes. 






Cauliflowers. 





The characteristic requirements of these various groups 
are as follows : - 

I. Principally phosphate with a liberal supply of 
potash and small amount of nitrogen. 

II. Principally potash with moderate amount of phos- 
phates and small amount of nitrogen. 

III. Principally nitrogen with liberal quantity of phos- 
phates and small amount of potash. For the more 
delicate plants of this group celery, asparagus, etc., 



200 



SOILS AND MANURES 



especially when they are required to be " blanched," the 
proportion of nitrogen should be much smaller than 
for the stronger plants like cabbages. Excess of nitrogen 
tends to produce strong colour and rankness of growth. 

IV. Phosphates and potash with small amount of 
nitrogen. The proportion of nitrogen for melons, to- 
matoes, etc., should be considerably larger than for 
peas and beans. 

According to the experiments of the French gardeners, 
mixed manures for the different groups should contain 
about the following percentages of the different ingre- 
dients, and should be applied at the rate of from two to 
four ounces per square yard. 

PERCENTAGE COMPOSITION OF MANURES FOR GARDEN CROPS. 



Group. 


Nitrogen. 


Phosphoric Ac-id. 


Potash. 




Per cent. 


Per cent. 


Per cent. 


I 


8 


14 


11 


II 


5 


10 


20 


Ill 


10 


14 


6 


IV 


4 


17 


18 



The proportion of potash in the manures for groups 
II. and IV. seems excessive, especially when the quanti- 
ties recommended to be used are considered. A great 
deal, of course, depends upon the kind of soil, and there 
is considerable difference of opinion on the subject. 

Strawberries are generally heavily dressed with farm- 
yard manure at the time of planting, and if the plants 
are renewed every three or four years there should be 
no necessity for any addition. If, however, the soil is 
poor or the plants are not frequently renewed some 
additional manure may be advantageously applied every 
year. Many gardeners dislike the use of stable manure 
as a top dressing for strawberries ; it fills them with 



THE PKINCIPLES OF MANUKING 201 

weeds, interferes with cultivation, and if left on the 
surface spoils the taste of the fruit. Truffont 1 recom- 
mends a dressing similar to that previously given for 
group IV. Other formulas recommend very different 
quantities, viz., (1) 8 parts of potassium nitrate and 
6 parts of superphosphate to be applied at the rate of 
6 cwts. per acre, and (2) 14 parts of nitrate of soda, 
6 parts superphosphate and 8 parts potassium chloride, 
to be applied at the rate of 12 cwt. per acre. The 
author prefers Truffont's recipe. 

Opinions differ also in regard to the importance of 
manures for fruit trees. Some authorities appear to 
think that they are of very little use. Others declare 
that " when fruit trees cease to bear, it is simply because 
they have exhausted all the fertilising ingredients around 
their roots and, that the best means of securing an 
abundant and continuous yield is to give the trees a 
dressing of manure every year." In the majority of 
cases manures have a beneficial effect on the yield from 
fruit trees though there are probably many causes which 
may counteract it. 

In general, the manures for fruit trees should consist 
mainly of phosphates and potash with a small proportion 
of nitrogen, as will be seen from the following recipes : 

1. On calcareous soils 

Superphosphate . . ,4 ozs. per square yard. 

Potassium chloride . . .2^ ,, ,, 

Nitrate of soda . . .1 ,, ,, 

2. On sandy soils 

Precipitated phosphate . .3 ,, ,, 

Potassium chloride . . 2^ ,, ,, 

Nitrate of soda . . .1 ,, ,, 

Gypsum OJ ,, 



J. K. A. S. E., vol. 63. 



202 SOILS AND MANURES 

3. On stiff soils- 
Basic slag .... 5 ozs. per square yard 
Sulphate of potash . . . 2 ,, ,, 

Sulphate of ammonia . .1 ,, ,, 

The dressings suitable for old trees might prove in- 
jurious to younger plants, and it is recommended that 
only about half quantities should be given until the 
trees are at least five years old, after which the quantities 
may be gradually increased. Some authorities recom- 
mend the addition of considerable quantities of calcium 
carbonate on all except the naturally limey soils. 

The requirements of flowers are very various, but in 
general they are similar to those of fruits, i.e., mainly 
phosphates and potash with small quantities of nitrogenous 
compounds. 

The composition of some mixed manures sold for 
garden purposes is given on p. 288. 

CLASSIFICATION OF MANURES. 

Artificial and Natural Manures. The term " artificial 
manure " is, by common consent, applied in a general 
way to all kinds of manufactured and prepared articles 
used for fertilising purposes. It is popularly under- 
stood and has been used in that sense in preceding, 
pages. If used for purposes of classification, it almost 
necessarily involves the adoption of the expression 
" natural manures " for other substances. Such a dis- 
tinction, however, is purely arbitrary ; it is often highly 
inconvenient and is apt to be misleading. No acceptable 
definition of the terms has been formulated and no 
general agreement as to their application has ever been 
reached. Farmyard manure is almost the only fer- 
tilising substance that has not been treated by some 
writers as artificial manure; guanos, bones, sludge and 



THE PEINCIPLES OF MANURING 203 

other substances which are generally regarded as arti- 
ficial, have often been treated as natural manures. As 
a popular expression it is useful and often convenient, 
but it is to be regarded as only vaguely descriptive and 
not as an exact definition. 

Light and Heavy Manures. The terms light and heavy 
were at one time introduced and used practically as 
substitutes for artificial and natural. Farmyard manure, 
seaweed, plant refuse, and all kinds of bulky organic 
substances which affect the physical properties of the 
soil, may be described as heavy manures in contra- 
distinction to the more concentrated light manures which 
affect the chemical properties only. These terms imply 
a distinction of some importance, and are therefore pre- 
ferable, but they never Jbecame popular and are now 
practically obsolete. 

General and Special Manures. The first of the above 
methods is a clumsy attempt to classify manures accord- 
ing to their origin, and the second is an attempt to 
classify them according to their uses. No classification 
can be altogether satisfactory that does not include both 
ideas. The latter is the more important, and should be 
made the primary basis of division. 

It has been shown (p. 181) that the chemical functions 
of manures are twofold, viz., restitution and addition of 
plant foods. Eestorative manures must be of a complete 
or general character, i.e., must contain all the fertilising 
ingredients. Additive manures must be adapted to the 
special requirements of the soils and crops to which they 
are applied. They may be, but are not necessarily or 
usually complete. Manures may therefore be conveniently 
divided first into 

1. General manures. 

2. Special manures. 

The essential characteristic of general manures is that 



204 SOILS AND MANURES 

they contain appreciable quantities of all three fertilising 
ingredients nitrogen, phosphoric acid and potash. They 
may be used as additive substances, but their more im- 
portant function is restitution. The so-called natural 
manures are, for the most part, general manures. They 
may be of organic origin, i.e., consist of animal or vege- 
table matter or a mixture of the two, or they may be 
wholly or partly mineral and may even be artificially 
prepared or manufactured. 

Special manures are not complete and only a few of 
them contain appreciable quantities of more than one 
fertilising ingredient. They are classified as nitrogenous, 
phosphatic or potash manures, according to the nature 
of the fertilising ingredient they contain. By combining 
two or more special manures, general manures can be 
produced and may be used for purposes of restitution. 
They are used, however, principally as additive sub- 
stances to satisfy the special requirements of the soils 
or crops to which they are applied. The so-called artificial 
manures are for the most part special manures. They 
may be of organic or of mineral origin, and may be 
subdivided accordingly. They are generally prepared or 
manufactured substances and are usually purchased, not 
produced on the farm. 

This classification of manures may be stated in tabular 
form thus: 

I. GENERAL MANURES. 

1. Of animal origin -- nitrogenous guanos, liquid 
manure, sewage manures, etc. 

2. Of vegetable origin green plants and plant refuse, 
seaweed, damaged feeding cakes, etc. 

3. Of mixed (animal and vegetable) origin farmyard 
manure, composts, night soils, etc. 

4. Of mineral origin mixed special manures, etc. 



THE PEINCIPLES OF MANUEING 205 

II. SPECIAL MANURES. 

1. Phosphatic manures mineral phosphates, phos- 
phatic guanos, prepared phosphates, etc. 

2. Phospho-nitrogenous manures bones, meatmeals, 
fish guanos, etc. 

3. Nitrogenous manures nitrogenous organic sub- 
stances, ammonium salts, nitrates, etc. 

4. Potash manures potash-bearing minerals, potas- 
sium salts, wood ashes. 

5. Miscellaneous manures common salt, sodium sili- 
cate, gypsum, magnesium salts, sulphate of iron, etc. 



CHAPTEK VIII 

PHOSPHATIC MANURES 

PHOSPHATES are by far the most popular and most 
extensively used of all the special or so-called artificial 
manures. The history of their introduction dates hack 
to the time towards the latter end of the 18th century 
when bones were first used in this country. The manurial 
value of bones was soon recognised, and in 1840 Liebig 
proposed to hasten their action by treating them with 
sulphuric acid. A couple of years later, Lawes applied 
the same process to coprolites and mineral phosphates, 
and took out a patent for the manufacture of what are 
now called superphosphates. Since that time the use of 
phosphatic manures has steadily increased, and enormous 
quantities are now consumed annually. It is scarcely 
too much to say that, in this country at least, phosphatic 
manures have come to be regarded as practically indis- 
pensable in modern farming, and that their use is con- 
sidered almost as natural as that of farmyard manure 
itself. 

NATIVE PHOSPHATES. 

Use of Native Phosphates as Manure. The native 
phosphates have been used to some extent, directly, as 
manures and it has been demonstrated that, when finely 
ground, they may have a distinctly favourable effect on 
the growth of crops. Owing to their comparative in- 
solubility, however, their action is necessarily slow, 
though it may be more rapid than was at one time 



PHOSPHATIC MANURES 207 

supposed. If applied in large quantities to poor land 
they might be useful as a kind of permanent improve- 
ment. Smaller quantities of some of the more active 
manufactured products would, however, give better im- 
mediate results, and if applied at intervals, would, in the 
end, be more profitable. The use of untreated native 
phosphates never became general for the ordinary pur- 
poses of farming and is now obsolete. Special interest 
attaches to them as the substances from which super- 
phosphates and other special manures are chiefly pre- 
pared, and it is necessary to give some account of them. 
General Properties. There are several kinds or 
varieties of native phosphate differing both in origin and 
in character. They occur in grains, small fragments, 
and in rock-like masses, in various parts of the world. 
Some are of organic origin, but the majority are purely 
mineral. The latter occur both in the crystalline and 
amorphous forms, and are often weathered and partly 
decomposed. They are all practically insoluble in pure 
water, but are slowly attacked by carbonic acid, and are 
fairly easily dissolved by strong acids. With the excep- 
tion of isome phosphates of iron and aluminium, which 
occur in considerable quantities but are not very widely 
distributed, they all consist essentially of normal ortho- 
phosphate of lime, called tricalcic phosphate. Some are 
met with in a very pure state and contain upwards of 
95 per cent, of tricalcic phosphate, but they are usually 
associated with larger or smaller quantities of quartz, 
calcium carbonate, calcium fluoride, oxides of iron, 
alumina, and other impurities, and the proportion of 
tricalcic phosphate is sometimes under 30 per cent. 

Production and Imports of Phosphates. The total pro- 
duction of phosphates in various parts of the world, in- 
creased from about half a million tons in 1880 to more 
than a million tons in 1890, and in 1900 it amounted 



208 



SOILS AND MANUKES 



to upwards of two million tons. The following tables 
show the quantities of phosphates imported into the 
United Kingdom from various countries in 1907, and the 
total amounts imported annually during the ten previous 
years : 

QUANTITIES OF PHOSPHATES IMPORTED INTO THE UNITED KINGDOM 
FROM VARIOUS COUNTRIES IN 1907. 

Country. Tons. 

Algeria . 47,047 

Belgium 37,602 

France 69,703 

Germany ......... 337 

Guiana (French) 4,491 

West Indies (Dutch) 9,572 

Netherlands 6,764 

Norway 657 

Tunis . 175,552 

United States of America 152,416 

Pacific Islands 1,530 

Total . . 505,671 



TOTAL QUANTITY OF PHOSPHATES IMPORTED INTO THE UNITED 
KINGDOM IN EACH YEAR FROM 1898 TO 1907. 



Year. 


Tons. 


Year. 


Tons. 


1898 


330,610 


1903 


392,782 


1899 


420,108 


1904 


419,270 


1900 


355,502 


1905 


420,988 


1901 


354,890 


1906 


442,970 


1902 


364,859 


1907 


505,671 



The native phosphates may be classed, according to 
origin as 

1. Apatites and phosphorites, 

2. Coprolites. 

3. Phosphatic guanos. 



PHOSPHATIC MANUEES 209 

Apatites and Phosphorites. The terms apatite and 
phosphorite must be regarded as practically interchange- 
able. The latter was originally applied to the less pure 
and amorphous forms, but seems now to be generally 
preferred for commercial purposes. 

According to the formula, 3 Ca 3 P 2 8 + Ca F 2 , given for 
fluor apatite (p. 20), the pure substance should contain 
92'25 per cent, of tricalcic phosphate and 7*75 per cent, 
of calcium fluoride. The following analysis, by Voelcker, 
shows the actual composition of a sample of Norwegian 
apatite : 

Per cent. 
Tricalcic phosphate . . . . . . . 90'07 

Calcium chloride 6' 13 



Calcium fluoride 
Oxide of iron 
Alumina 
Potash and soda 
Water . 



2-54 
0-29 
0-38 
0-17 
0-42 



100-00 



Apatite is found massive in several countries. Con- 
siderable quantities were at one time shipped from Canada 
and Norway, but since the discovery of the Florida phos- 
phate the demand for these products has largely 
decreased. 

Phosphorites are found in veins, usually mixed with 
quartz, in different rocks ; as nodules embedded in lime- 
stones and sandstones ; as a connective cement in 
breccias ; as kidney-shaped stalactites ; and in the form 
of the black phosphatic slate of the coal measures. In 
the natural state these different deposits of phosphorite 
vary widely in richness. Some samples contain upwards 
of 60 per cent., and others less than 20 per cent, of 
tricalcic phosphate, but they are generally subjected 
to processes of purification and graded before being 

S.M. p 



210 SOILS AND MANURES 

put on the market. The percentage of phosphate in 
commercial samples is therefore much higher and often 
compares favourably with that of samples described as 
apatites. 

Spanish phosphorite, often called Estremadurite, after 
the name of the province from which it is obtained, was 
formerly regarded as one of the most important varieties, 
but Algerian phosphate seems to have largely displaced 
it from the market. The latter closely resembles the 
Spanish phosphorite in some respects, but is of better 
quality. Average samples contain about 70 per cent, of 
tricalcic phosphate. 

French, German and Portuguese phosphates have fre- 
quently been classed together as products of similar but 
inferior quality. Some of them are comparatively rich 
in phosphates, but they are usually mixed with quantities 
of clay and marl, and contain a considerable proportion 
of iron and alumina which greatly reduces their value 
for the manufacture of superphosphates. 

The German, commonly called Nassau or Lahn phos- 
phate, contains from 35 70 per cent, of tricalcic phos- 
phate. 

The French, or Lot phosphate, which is exported from 
Bordeaux, and the Portuguese phosphate are both poorer 
than the German. 

South Carolina (Charleston) and the more recently 
discovered Florida phosphates are similar in character, 
and until the introduction of Algerian phosphates were 
by far the most important source of the world's supply. 
The phosphate consists of nodules embedded in the rock, 
and is considered by some to be similar to the Lot phos- 
phate. The deposits are of Eocene age, consist of beds 
of marl perforated by mollusca, and contain fossil 
bones and teeth of sharks, etc. There are two varieties, 
called river phosphate and land phosphate respectively. 



PIIOSPHATIC MANUEES 211 

The former is obtained by dredging the river beds, and 
the latter by mining ashore. The river phosphate gener- 
ally contains a somewhat larger proportion of iron and 
alumina than the land phosphate, but in other respects 
they are much alike. The South Carolina deposits were 
discovered about forty years ago, and those of Florida 
some twenty years later. Both have proved enormously 
productive. The Florida phosphates are much the richer 
of the two. The land phosphate contains 70 to 80 per 
cent., and the river phosphate about 60 per cent, of tri- 
calcic phosphate. The best quality of Charleston phos- 
phate, after grading, contained only 60 per cent. 

Belgian and Somme phosphates differ from those pre- 
viously mentioned in their mode of occurrence. On the 
Franco-Belgian frontier there is found a great deposit 
of friable phosphatic chalk called " craie-grise," lying 
on a bed of ordinary white chalk. It extends over an 
area of some seven million acres and is chiefly worked 
near the town of Mons, in Belgium, and in the Somme 
and Pas de Calais departments of France. The phosphate 
exists in the form of yellow crystalline grains embedded 
in the chalk matrix which, in the crude state, contains 
from 20 to 30 per cent, of tricalcic phosphate. In the 
upper layers pockets are often found containing a larger 
proportion of this phosphatic sand and less calcium 
carbonate than the main bulk. These pockets are found 
chiefly on the French side. The chalky matter is much 
softer than the phosphatic grains and a large proportion 
of it can be got rid of by processes of grinding and sifting. 
The proportion of phosphates is thus raised far above 
that found in the crude product. The Sornme phosphate 
is graded and sold in different qualities varying from 
60 to 80 per cent. It should not be confused with the 
Bordeaux or Lot phosphate, though both are sometimes 
naturally spoken of as French. The Belgian phosphate 

p 2 



212 SOILS AND MANURES 

is generally of inferior quality. The best qualities are, 
as a rule, poorer than the lowest grades of Somme phos- 
phate and rarely contain more than about 40 per cent, 
of tricalcic phosphate. 

Caribbean or West Indian phosphates are sometimes 
described as phosphatic guanos. Some of them are 
properly so-called, being derived from guanos, but others 
are of purely mineral origin. Deposits are found on 
several of the islands and are named accordingly. They 
have been extensively worked, and some of the richest 
were exhausted soon after they were discovered. Eedonda 
and Alta Vela phosphates are not phosphates of lime but 
phosphates of iron and alumina. The former contains 
only about 3 per cent, and the latter 11 per cent, of lime. 

Quantities of phosphates of fairly good quality are 
also produced in various parts of South America, par- 
ticularly Venezuela, Guiana, and Brazil. 

Coprolites. The name coprolites was first given by 
Dr. Buckland to certain peculiar stones found in the 
Gloucestershire lias, which had long been familiar and 
were locally known, from their appearance, as fossil fir- 
cones. .He described them as oblong pebbles, usually 
about two to four inches long and half as thick some of 
them are much larger varying in colour from ash grey to 
dark brown or black. They exhibit a conchoidal fracture, 
and the peculiar structure which shows that they have 
passed through animal intestines. They appear to con- 
sist of the fossilised excrement of certain extinct reptiles 
called Ichthyosauri, which infested the low-lying swampy 
ground of Cambridgeshire and the eastern counties. The 
term coprolite is derived from KOTT^OS (dung) and At0o? 
(stone), literally dtingstone. They are found mixed with 
the bones and teeth of fishes and sometimes even the 
remains of the smaller species of Ichthyosauri. They 
consist mainly of calcium phosphate, but are usually 



PHOSPHATIC MANURES 213 

associated with larger or smaller quantities of calcium 
carbonate. 

Quantities of phosphatic nodules are also found in 
the Suffolk Crag and in various parts of the Green Sand. 
They closely resemble the coprolites in appearance, and 
were at one time thought ~fco be of the same nature, 
but are now known to be concretionary. They are also 
very similar in chemical composition, but generally con- 
tain a larger proportion of iron and alumina. They have 
been called pseudo - coprolites to distinguish them from 
those of fsecal origin, but the term coprolite is generally 
applied without discrimination to all kinds of phosphatic 
nodules. The composition of coprolites is necessarily 
somewhat variable, but they usually contain from 50 to 
60 per cent, of tricalcic phosphate. The following 
analysis shows the proportions of various ingredients 
found in a particular sample: 

Per cent. 

Tricalcic phosphate 54'0 

Calcium carbonate ....... 28' 1 

Oxide of iron and alumina ...... 7'4 

Silica ' 0-7 

Organic matter ........ 2*0 

Water 3'9 

Not estimated . 3'9 



100-00 

Coprolites are found chiefly in Cambridgeshire, Suffolk 
and Bedford, and to some extent in Norfolk, Buckingham 
and Essex. They are also found in France, in the 
neighbourhood of Boulogne, in Eussia, and some other 
countries. Like apatites and phosphorites they are used 
for the manufacture of superphosphates, and as much 
as 30,000 tons have been produced in England in a 
year, a single "acre yielding sometimes as much as 300 



214 SOILS AND MANURES 

tons. The English coprolites, especially those from Cam- 
bridgeshire and Suffolk, are generally richer than the 
French. 

Phosphatic guanos. The true phosphatic guanos, like 
the true coprolites, are of organic origin. They are 
derived from guano by the gradual removal of the volatile 
and soluble matters (p. 295), and consist of the insoluble 
phosphatic residue. Traces of alkalis and sometimes even 
of nitrogen are occasionally present, but the quantities 
of these substances are negligible and the material is to 
be regarded simply as a native phosphate. Phosphatic 
guanos are usually very rich. Commercial samples con- 
tain from 70 80 per cent, of tricalcic phosphate. They 
are comparatively easily ground and dissolved, and as 
they contain but little iron and aluminium, are very 
suitable for the manufacture of high class superphos- 
phates. Apart from these qualities, they are of no greater 
value than apatites and phosphorites of similar composi- 
tion notwithstanding their organic origin. 

They are found in several of the Pacific and West 
Indian Islands, and in various parts of North and South 
America. 

SUPERPHOSPHATES. 

The native phosphates, it has been said, are not largely 
used as manures, in the natural state. Their action is 
too slow, and it is found more profitable to hasten it by 
treatment with sulphuric acid. The products are called 
superphosphates. The name is a purely conventional 
one and does not imply, as might perhaps be supposed, 
that they contain more phosphoric acid than the native 
or other phosphates. The treatment with sulphuric acid 
does not alter the quantity of phosphoric acid present. 
AH that it does is to convert the insoluble tricalcio 



PHOSPHATIC MANURES 215 

phosphate into the soluble monocalcic compound accord- 
ing to the equation : 

Ca 3 (P0 4 ) 2 + 2H 2 S0 4 = 2CaS0 4 + CaH 4 (P0 4 ) 2 

Tricalcic Sulphuric Calcium Monocalcic 

phosphate. acid. sulphate. phosphate. 

As the calcium sulphate is not removed, the super- 
phosphate necessarily contains a smaller proportion of 
phosphoric acid than the untreated phosphate. 

If a smaller quantity of sulphuric acid were used, 
dicalcic phosphate would be formed, thus 

Ca 3 (P0 4 ) 2 + H 2 S0 4 = CaS0 4 + 2CaHP0 4 

Tricalcic Sulphuric Calcium Dicalcic 
phosphate. acid. sulphate. phosphate. 

If a larger quantity of sulphuric acid were employed, 
the whole of the lime would be removed and phosphoric 
acid would be formed, thus 

Ca 3 (P0 4 ) 2 + 3H 2 S0 4 = 3CaS0 4 + 2H 3 P0 4 

Tricalcic Sulphuric Calcium Phosphoric 
phosphate. acid. sulphate. acid. 

Tricalcic phosphate is the normal " phosphate of lime," 
and is the substance commonly alluded to by that name. 
Dicalcic phosphate is insoluble in water, and though more 
readily soluble in various reagents than the tricalcic 
form, it does not exhibit the characteristic properties of 
a superphosphate. Monocalcic phosphate is soluble in 
cold water, but if the solution be heated, or even on 
standing in the cold, it changes into dicalcic phosphate 
and phosphoric acid, thus 

CaH 4 (P0 4 ) 2 = CaHP0 4 + H 3 P0 4 

Monocalcic Dicalcic Phosphoric 

phosphate. phosphate. acid. 



216 SOILS AND MANURES 

Phosphoric acid, properly so-called, is the hydrate H 3 P0 4 , 
but the term is often applied to the oxide P 2 0s. 

The relation of these various compounds to each other 
may be illustrated as follows : 

H 2 0) H 2 0) CaO) CaO) 

P 2 5 H 2 0[P 2 5 CaO P 2 5 H 2 OlP 2 5 CaOfP 2 5 
H 3 0) H 2 0] CaO) CaO) 

Phosphorus Phosphoric Monocalcic Dicalcic Tricalcic 

pentoxide. acid. phosphate. phosphate. phosphate. 

H 3 P0 4 CaH 4 (P0 4 ) 2 CaHP0 4 Ca 3 (P0 4 ) 2 

Manufacture of Superphosphates. It is unnecessary to 
enter into technical details, but a brief outline of the 
process is germane to the subject. The object is to con- 
vert the insoluble tricalcic phosphate into the soluble 
monocalcic form, and to avoid as far as possible, the 
production of dicalcic phosphate and of phosphoric acid. 
Dicalcic phosphate, being insoluble, is but little better 
than the original substance. If much free phosphoric 
acid be present the substance is apt to get into a moist 
and sticky condition in which it is difficult to handle, and 
which is favourable to reversion (p. 219). 

A weighed quantity of the finely ground raw (native) 
phosphate is introduced into an apparatus called the 
" mixer"; a measured quantity of cold dilute 1 sulphuric 
acid is run in and agitated until the two are thoroughly 
mixed together. The whole is then dropped into a pit 
or den, as it is called, and allowed to remain until the 
reaction is complete. 

The mixer generally consists of an oblong wooden box, 
lined with lead, through the middle of which runs a 

1 The strength of the acid is a matter of considerable technical 
importance. The " chamber acid " commonly used has a specific 
gravity about 1'5 r@. 



PHOSPHATIO MANURES 217 

revolving shaft with spikes attached to it for stirring up 
the materials. 

The reaction between the acid and the phosphate does 
not take place in the mixer, and the slight rise of tem- 
perature which occurs is due mainly to the action of the 
acid on carbonates, chlorides and fluorides which are 
commonly associated with the native phosphates. When 
the proportion of such impurities is large, a considerable 
quantity of acid is used up in this way, and samples 
which contain them are, therefore, of lower commercial 
value. The additional quantity of calcium sulphate formed 
by these reactions may, however, be useful as a drying agent. 

The mixing process does not occupy more than a few 
minutes, and, when it is complete, the mixture, which is 
of a thin fluid consistency, is run off. It is in the pit or 
den that the principal reaction occurs, and the tempera- 
ture rises to over 100 deg. C. When it is over, the mass 
gradually cools down, the calcium sulphate absorbs the 
water and the whole sets into a solid mass which requires 
to be dug out with picks, but is found to be, if properly 
made, in a very friable, easily powdered condition. 

Composition and Value of Superphosphates. It has 
been shown that superphosphates consist essentially of a 
mixture of monocalcic phosphate and calcium sulphate. 
As it is not practicable to completely dissolve the whole 
of the phosphate, a certain proportion usually from two 
to three per cent. of tricalcic phosphate is always present. 
Dicalcic phosphate and free phosphoric acid are also 
commonly present, but as a rule, only in small propor- 
tion. They may be formed in process of manufacture, 
or subsequently from the monocalcic phosphate. Sand, 
compounds of magnesia, iron, aluminium, and other 
impurities originally present in the raw phosphate or in 
the acid, are of course found in the manufactured pro- 
duct. The proportion of sand and insoluble matters, 



218 SOILS AND MANURES 

apart from gypsum, may amount in some cases to from 
10 to 20 per cent. 

The value of superphosphates obviously depends upon 
the proportion of soluble phosphates they contain. The 
standard quality is one which contains about 11*5 per 
cent, of phosphoric acid in a soluble state, equal to 
25 per cent, of phosphate of lime rendered soluble. 
Analyses of superphosphates are generally expressed in 
this form. The phosphate of lime here referred to is, 
of course, tricalcic phosphate, and the expression " phos- 
phoric acid" means the oxide (P 2 5 ), not the hydrate 
to which the name is more properly applied. It is 
an unfortunate misnomer, and occasionally leads to 
some confusion, but as it is in general use it must be 
accepted. It is noteworthy that neither the P 2 05 nor 
the tricalcic phosphate are present in the superphosphate 
as such. The former occurs as a compound, and the 
latter not at all, but when the quantity of P 2 5 has been 
found it is easy to calculate how much tricalcic phosphate 
it is equivalent to. 

Superphosphates of higher grade, i.e., containing up 
to 35 per cent, or even more, of soluble phosphates are 
sometimes prepared. Probably owing to the competition 
of other manures rich in phosphates, they have become 
much more common of late years. A very high grade 
product called "double superphosphate," which contains 
about 80 per cent, of soluble phosphate, has been manu- 
factured chiefly in Germany and the United States. 
Apart from practical difficulties it is obvious that such a 
product could not be obtained, even theoretically, in the 
ordinary way. In the special process employed for this 
purpose the purest native phosphates are selected, and 
treated with excess of sulphuric acid so as to liberate 
the phosphoric acid ; the calcium sulphate is removed, 
and more tricalcic phosphate added to the free phosphoric 



PHOSPHATIC MANURES 219 

acid which remains, until it is practically all converted 
into monocalcic phosphate according to the following 
equation : 

Ca 3 (P0 4 ) 2 + 4H 3 P0 4 = 3CaH 4 (P0 4 ) 2 

All that is accomplished by the process, therefore, is the 
elimination of the calcium sulphate, and under ordinary 
circumstances, the advantages of this extreme concentra- 
tion are not apparent. 

Three and a-half tons of 25 per cent, quality contain 
exactly the same amount of phosphates as two and a- 
half tons of 35 per cent, quality, and have precisely the 
same fertilising power. If they can be obtained at the 
same rate per unit of phosphate, the latter would be 
the more economical to the extent of the saving in the 
cost of transport and handling. Apart from this, there 
is no difference between the two. The price is generally 
proportional to the percentage of soluble phosphate, but 
standard qualities can, as a rule, be obtained at lower 
rates than those which have to be specially prepared. 

Reverted Phosphates. When superphosphates are kept 
for some time, the soluble phosphate gradually changes 
back into an insoluble form called reverted or reduced 
phosphate. Eeversion has been attributed to the forma- 
tion of dicalcic phosphate by the interaction of the mono- 
calcic and tricalcic compounds. Superphosphates do not, 
however, contain enough undissolved tricalcic phosphate 
to account for the extent to which the change has been 
observed to take place, and also on other grounds, the 
explanation is improbable. It is possible, however, that 
reversion may be due, in some cases, to the production 
of dicalcic phosphate from monocalcic phosphate itself 
according to the equation : 

CaH 4 (P0 4 ) 2 = CaH 4 P0 4 + H 3 P0 4 

Monocalcic Dicalcic Phosphoric 

phosphate, phosphate. acid, 



220 SOILS AND MANURES 

This change is known to occur when a solution of 
monocalcic phosphate is heated, and also at the ordinary 
temperature on prolonged standing, and it is quite pos- 
sible that it may occur in superphosphates if allowed to 
become damp or heated. 

Keversion may also be caused by the interaction of 
monocalcic phosphate with salts of iron and aluminium 
chiefly sulphates formed during the process of manufacture 
of the superphosphates, thus 

CaH 4 (P0 4 ) 2 + A1 2 (S0 4 ) 3 = 2A1P0 4 + CaS0 4 + 2H 2 S0 4 

Monocalcic Aluminium Aluminium Calcium Sulphuric 
phosphate. sulphate. phosphate, sulphate. acid. 

Pyrites and silicates of iron and aluminium, if present 
in the raw material, are not attacked by the sulphuric 
acid or subsequently acted upon by the monocalcic phos- 
phate in the superphosphate, and are therefore harmless. 
But oxides and phosphates of iron and aluminium, if 
present in the raw phosphates, are converted into sul- 
phates by the action of the sulphuric acid and afterwards 
cause the soluble phosphate to revert as above described. 

This is probably the principal cause of reversion. It 
is for this reason that native phosphates which contain 
much iron and alumina, e.g., Bedonda phosphate, are 
deemed unsuitable for the manufacture of superphos- 
phates. Samples which are destitute of compounds of 
iron and alumina or which contain only the silicates of 
these bases, or pyrites, if dry, can be kept for long periods 
without reversion taking place. The reversion of the 
soluble phosphate to the insoluble state greatly reduces 
the value of the superphosphates. The reverted phosphate 
is, however, more readily soluble than the original tricalcic 
compound, and has consequently a higher agricultural 
value. It is largely soluble in a neutral solution of ammo- 
nium citrate, and is often called the citrate soluble 
phosphate. 



PHOSPHATIC MANUEES 221 

Action of Superphosphates as Manure. When super- 
phosphate is applied to the soil, the monocalcic phosphate 
probably passes into solution, and on contact with calcium 
carbonate is reprecipitated in the tricalcic form, thus : 
CaH 4 (P0 4 ) 2 + 2CaC0 3 = Ca 3 (P0 4 ) 2 + 2C0 2 + 2H 2 

Monocalcic Calcium Tricalcic Carbon Water, 

phosphate. carbonate, phosphate. dioxide. 

The product is, by this means, thoroughly disseminated 
through the soil, in a state of infinitely fine division 
far finer than can be produced by any mechanical process 
of grinding and is more easily soluble than the original 
phosphate. The superiority of superphosphates, as com- 
pared with the untreated native phosphates, must be 
attributed to these conditions. It is obvious that if pre- 
cipitation take place before the substance is applied, the 
phosphate cannot be so intimately mixed with the soil, 
and, therefore, even if it were equally soluble, could not 
be expected to produce such good results. 

Some years ago a patent was taken out for the pre- 
paration of a new manure called basic superphosphate 
which was expected to compare favourably with basic 
slag owing to the more ready solubility of the phosphate. 
It was prepared by mixing a quantity of lime with 
ordinary superphosphate, but it seems obvious that it 
would be more advantageous to apply the lime separately 
to the soil so that the precipitation might take place within 
the soil and not before application. 

In the absence of a sufficient amount of lime in the soil 
the soluble phosphate would probably be converted into 
phosphates of iron and alumina (p. 133). These com- 
pounds, it has been said, are less readily soluble in dilute 
acids than phosphate of lime when freshly precipitated, 
and they probably undergo gradual dehydration and 
become still less soluble. The best effects are therefore 
obtained from superphosphates when applied to soils 



222 SOILS ;AND MANUEES 

which are at least moderately well supplied with lime, 
and in its absence the residual value of the manure will 
probably be considerably diminished. 

Superphosphate is pre-eminently suitable for the growth 
of root crops and especially of turnips and swedes. It 
may be used for pastures, leguminous crops or cereals if 
required. The strongly acid character of the manure has 
a tendency to promote the development of " linger and toe " 
in turnips, to discourage the growth of clover in pastures, 
and to render it harmful, rather than beneficial to all 
leguminous crops. The tendency to produce such effects 
is more marked in the more concentrated varieties, and 
is, of course, much greater when large quantities of the 
manure are used. It is not noticeable at all except in 
soils that are deficient in lime. 

In general, superphosphate is the best kind of phos- 
phatic manure for the lighter class of soils. It is cer- 
tainly by far the best for those that are inclined to 
dryness. It is commonly applied at the rate of from 
2 to 5 cwt. per acre. Sometimes larger dressings are 
used, but the smaller quantity is, as a rule, sufficient. 
It is sometimes applied in the back end of the year, but 
for most purposes better results are obtained by spring 
sowing. 

Effects of Mixing Superphosphates with other Manures. 
For the reasons given above it is inadvisable to mix 
superphosphates with any form of lime, basic slag, wood 
ashes, salt, or in short, any substance which can react 
with the monocalcic phosphate and cause precipitation. 
Above all, it should not be mixed with nitrate of soda or 
similar salts. To do so may not only spoil both manures, 
but also bring about results of a highly dangerous 
character. Heat is developed by the reaction, noxious 
fumes (oxide of nitrogen) are given off, and may become 
a menace both to life and property. 



PHOSPHATIC MANUKES 223 

The change may be represented by the following 
equation l : 





3CaH 4 (P0 4 ) 2 

Monocalcic 
phosphate. 


+ 8NaN0 3 = 

Nitrate of 
soda. 




Ca 3 (P0 4 ) 2 H 

Tricalcic 
phosphate. 


h 4Na 2 HP0 4 - 

Sodium 
phosphate. 


h 4NA + 2 2 -i 

Nitric Oxygen, 
peroxide. 


- 4H 2 

Water. 



No great harm is likely to result from mixing small 
quantities of these two substances if it be done in the 
open air immediately before they are applied to the 
land. At the worst it can only spoil the superphosphate 
by causing it to revert to the insoluble state. The only 
advantage of mixing the manures is a slightly greater 
facility of distribution, and it is doubtful whether this is 
not more than neutralised by the labour of mixing. At any 
rate, one strong argument against mixing them is that they 
should not, as a rule, be applied at the same time of 
year. Superphosphate should be applied some time 
before the seed is sown, and nitrate of soda not until 
after it has germinated. Superphosphate may, if desired, 
be mixed with sulphate of ammonia. These manures 
do not act upon each other and there is less reason for 
not applying them at the same time than in the case 
of nitrate of soda. For those who are at all uncertain 
of the chemistry of the subject, the only safe rule is not 
to mix superphosphate with anything before applying 
it to the land, but there is no reason why any manure 
should not be applied to the same soil that has already 
received a dressing of superphosphate. It may also be 
safely mixed with sulphate of potash and a mixture of 
this kind can be obtained commercially. It is called 
" Potassic superphosphate," and contains about 23 per 

1 The reaction may be more complex than is shown here. 



224 SOILS AND MANUBES 

cent, of soluble phosphates and 4 per cent, of potash. 
It is convenient for certain purposes, especially when only 
small quantities are required. Equivalent quantities ' of 
superphosphate and sulphate of potash applied separately, 
give equally good results, and if they can be obtained 
at cheaper rates, are more economical. 

The quality of a sample of superphosphate cannot 
be judged by the colour, smell or other outward pro- 
perties. All that can be done by simple inspection is to 
determine whether it is in good sowable condition. 
Beyond this, the sole criterion of quality is the proportion 
of soluble phosphates it contains, and that can only be 
determined by chemical analysis. 

BASIC SLAG.. 

Basic slag, basic cinder, Thomas' phosphate meal or 
powder, for it is known by all these names, is a substance 
of entirely different origin and character. It is a bye- 
product from steel works, and has been known only for a 
comparatively short time. The process by which it is 
produced is of a highly technical character, but the main 
outlines are easily followed. 

In the well-known Bessemer process for the manu- 
facture of steel, the crude product from the blast fur- 
naces, called " pig iron," is melted, and air is blown 
through the molten mass in order to oxidise the carbon. 
The operation is performed in a specially constructed 
apparatus known as the " converter " a pear-shaped iron 
vessel lined with infusible fireclay. As originally devised, 
the process could not be applied to samples that con- 
tained much phosphorus because the oxide of that ele- 
ment, being non- volatile, does not escape like that of 
carbon, and its presence affects the properties of the steel 
in such a way as to make it useless for many purposes 




FIG. 29. 




S.M. 



FIG. 30. 



226 SOILS AND MANURES 

This difficulty has been surmounted by the introduction 
of a basic lining consisting of lime, or dolomite powder 
a mixture of lime and magnesia and the addition of 
lime to the molten pig iron. With this improvement, intro- 
duced by Thomas and Gilchrist in 1879, the process can be 
applied to products containing from 2 to 4 per cent, of 
phosphorus, and it is the spent slag from this part of the 
process that is ground up and sold as manure. 

When the converter has been properly lined and pre- 
pared, the molten pig iron is introduced, and the air 
blast started. The crude product may contain from 
3 to 6 per cent, of carbon, and from 1 to 2 per cent, 
of silicon, besides phosphorus, sulphur and other im- 
purities, all of which, together with some of the iron 
and manganese, undergo oxidation. A considerable rise 
of temperature takes place in consequence, the lime melts 
and unites with the oxides of phosphorus and silicon, and 
the products, being lighter than the iron, rise to the 
top. The carbon monoxide, formed by the oxidation of 
the carbon, escapes and becomes ignited on contact with 
the air, and the characteristic flame of the gas is seen 
playing on ttie surface until the iron is completely de- 
carbonised. The operation, which only occupies about 
15 minutes, is then stopped and the slag is poured off. 

In the illustration l the converter is shown turned up 
(Fig. 29) with the " blow " in operation; Fig. 30 shows 
the converter turned down after the "blow" is finished, 
and the slag being poured into the " ladle " below. When 
cold it presents the appearance of a hard massive black 
cinder. In order to prepare it for use as manure, it is sub- 
jected to a very elaborate process of grinding, for which 
special machinery has been invented. It is first broken 
into small pieces under stamps, then crushed under heavy 

1 Copy supplied by Chemical Works, late H. & E. Albert, London. 



PHOSPHATIC MANUKES 227 

rollers, sifted to remove pieces of iron, and finally ground 
in powerful roller-mills to the state of fine powder in which 
it is ordinarily placed on the market. 

Chemical Composition. The following complete 
analysis x shows the proportions of the various con- 
stituents in a typical sample. 2 

Per cent. 
Calcium oxide . . . . . . . 41*58 

Magnesium oxide . . . . . . .6*14 

Aluminium oxide ........ 2*67 

Ferric oxide 8'54 

Ferrous oxide 13 '62 

Manganous oxide . . . 3'79 

Vanadium oxide ........ 1'29 

Silica 7-38 

Calcium sulphide . . . . . . . . 0'54 

Sulphur trioxide . . . . . . . 0'12 

Phosphorous pentoxide 14'36 

99-93 

The composition, however, is very variable. It depends 
primarily upon the composition of the crude iron, and 
to some extent also, upon the composition and quantity 
of the materials used to purify it. 

The proportion of phosphates in the final product 
will obviously be greater or less according to the amount 
of phosphorus in the crude iron, and it may be increased 
by the presence of phosphates in the materials added to 
the pig iron in the converter. In some cases phosphatic 
lime has been purposely employed in order that the resulting 
slags may show a high percentage of phosphates. This 
greatly increases their commercial value, but unless the 

1 Journal of the Iron and Steel Institute, 1887. 

2 Owing to the introduction of silica to increase the solubility and 
other recent improvements in manufacture the composition of he 
slags now produced differs slightly in some respects from that given 
above. 

Q 2 



228 SOILS AND MANUEES 

phosphate thus added is converted into the basic form, 
which is the special characteristic of the slag phosphates, 
its manurial value will be no greater than that of the native 
tricalcic phosphates. This, perhaps, may partly account 
for the differences in the solubility of the phosphates of 
slags obtained from different sources. The extraordinary 
differences sometimes observed in the fertilising power of 
slags containing approximately the same proportion of 
phosphates may be due to the same cause. 

The average quantity of lime employed is about one 
part to five or six of pig iron, but by using a smaller 
quantity, a product much richer in phosphates can be 
obtained. This necessitates subjecting the iron to a 
second treatment in order to complete the dephosphorisa- 
tion, but as only a small amount of impurity is left in 
the iron, the lime does not suffer much change in the second 
treatment and can be used over again. 

Under ordinary circumstances, the proportions of the 
principal constituents may vary within the following 
limits : 

Per cent. 

Lime 4060 

Oxides of iron ........ 10 20 

Silica 515 

Oxide of manganese 3 6 

Magnesia ......... 2 6 

Alumina ......... 1 3 

Phosphoric acid (P 2 O 5 ) . . . 1020 

The average proportion of phosphoric acid is about 
16 per cent., which is equal to 35 per cent, of tricalcic 
phosphate, but commercial samples containing as little 
as 25 per cent, and as much as 45 per cent, of phos- 
phates are quite common. The average of English makes 
is about 30 per cent. 

More than two-thirds of the total lime (calcium oxide) 
are always present in combination with phosphoric acid 



PHOSPHATIC MANUEES 229 

and silica, and the remainder usually only a small amount 
occurs in the free or uncombined state, i.e., as lime. 
The larger the proportion of phosphoric acid, silica, and 
other ingredients which unite with the lime, the smaller 
will be the proportion left in the free or uncombined state. 

When the proportion of iron, silica, etc., is large, that 
of the phosphoric acid, lime, and other ingredients is 
correspondingly diminished and rice versa. A large 
proportion of iron adds considerably to the difficulty of 
grinding, but has not been found otherwise objectionable. 

Basic Character of Slag Phosphates. An extremely 
important point in connection with the composition of 
basic slag is the character of the phosphate it contains. 
It may be amorphous or crystalline. When slowly cooled 
it naturally assumes the latter form, and it was by 
examination of the crystals that Hilgenstock discovered 
that it belongs to the class of substances known as basic 
salts, i.e., salts which contain a larger proportion of 
basic oxide than is normally required to combine with 
the acid oxide. 

In ordinary tricalcic phosphate the normal salt the 
ratio of lime (CaO) to phosphoric acid (P 2 5 ) is 1/18 
to 1, but in slag phosphates it is 1*57 to 1. It is not 
simply a mixture of free lime with ordinary tricalcic 
phosphate, but appears to be a genuine chemical com- 
pound in which the additional lime enters into the com- 
position of the molecule and cannot be distinguished 
from the other parts of lime present. The proportion of 
lime corresponds, in ordinary tricalcic phosphate, to the 
formula P./) 5 ,3CaO and in slag phosphates to the formula 1 
P 2 5 ,4CaO. The latter is therefore called tetracalcic, or 
basic phosphate of lime. 

The question is one not merely of scientific interest, 

1 The exact constitution of the basic phosphate is not known ; it 
is probably very complex, but the difference between the molecular 



230 SOILS AND MANURES 

but is of great practical importance because it accounts, 
in large measure, for the peculiar properties upon which 
the manurial value of slag depends. Basic salts are 
potentially bases, and exhibit the characteristic properties 
of the class, of which the most important is a capacity 
to react with acids. They are therefore generally much 
more active than the corresponding normal salts. This is 
conspicuously the case in regard to basic phosphate of 
lime. It is practically insoluble in pure water, but is 
attacked by water containing carbonic acid in solution. 
It is soluble to a very large extent in neutral citrate of 
ammonia, and in dilute citric acid solutions. The native 
(tricalcic) phosphates are practically insoluble in these 
reagents. The manurial effects of basic slag seem to 
show that the phosphates are more readily available to 
the plants than those of the native phosphates. This 
is probably due to the greater solubility which apparently 
depends upon the basic character of the phosphates. 

It would be a mistake to suppose that basic slags are 
so-called because they contain basic phosphates. The 
name was originally applied, not to Thomas' phosphate 
at all, but to the raw substance, in contradistinction to 
the acid slags which are also employed in iron-works, 
The latter generally consist of silica, and are used 
chiefly as a flux for basic impurities. They contain 
no phosphates, and are of no value for manurial pur- 
poses. Thomas' phosphate, it will be seen therefore, 

structure of the tricalcic and tetracalcic phosphate may be indicated 
as follows : 

O O 



Tricalcic phosphate. 

O O 

-O-Ca-O-Ca-O 

Tetracalcic phosphate. 



PHOSPHATIC MANUKES 231 

possesses a doubly basic character. It contains basic 
(tetracalcic) phosphates, and it also possesses the basic 
character of the original slag in virtue of the excess of 
lime which remains in the free or uncombined state. 

The amount pf " free lime " in slags as now produced 
rarely exceeds 6 per cent, and in some cases is less than 
2 per cent., 1 but a considerable proportion of that which 
is actually in combination with phosphoric acid and silica 
acts potentially as a base and can liberate ammonia from 
neutral salts. 

Manurial Effects. Basic slag acts best on clay soils ; 
these generally contain plenty of moisture, and owing 
to the comparative insolubility of the phosphates, 
this appears to be indispensable for the proper action 
of basic slag. Clay soils again are often deficient in lime, 
and though the total quantity of lime in an ordinary 
dressing of slag is small, it is finely ground, mixes well 
with the soil, and produces both physical and chemical 
changes (p. 113) which are of great benefit to the crops. 
Basic slag may give very good results on both sandy and 
humous soils, but not if they are at all dry, and it has even 
been found beneficial on chalky soils if the natural deficiency 
of potash is made good. It is particularly valuable as a 
means of supplying a readily available phosphate to soils 
that are inclined to acidity, and therefore to anbury, or 
other troubles arising directly or indirectly from that cause. 

Basic slag has been very largely used as a dressing for 
pastures in which it tends to encourage the growth of clovers, 
probably owing to its limey and basic character. The 
improvement in quality is often more striking than the 
increase in quantity of the herbage. Of course, on poor 
land or even in soils of moderate fertility, basic slag or 
other phosphatic manure will generally give a profitable 
increase. For the same reason it is perhaps the most 
1 Hendrick, J.S.C.I., July 1909. 



232 SOILS AND MANURES 

suitable form of phosphatic manure for other leguminous 
crops. 

Comparison of Basic Slag and Superphosphate. As a 
purely phosphatic manure, Thomas' phosphate naturally 
lends itself to comparison with superphosphate. In 
making such a comparison, however, it should be re- 
membered that, owing to the difference in their character, 
the same conditions are not equally suitable for both. 

From any point of view there is little to be gained by 
comparing the manurial effects of equal weights of 
Thomas' phosphate and superphosphate, unless they con- 
tain nearly the same proportion of phosphates. A ton 
of basic slag (35 per cent.) contains approximately the 
same quantity of phosphoric acid as 28 cwt. of super- 
phosphate (25 per cent.), and it is a matter of consider- 
able scientific interest to determine in which of these 
two forms, i.e., as acid phosphate or as basic phosphate, 
a given quantity of phosphoric acid will produce the best 
result. Numerous experiments have been carried out in 
order to settle this point, but have given contradictory 
results, probably because it is very difficult to secure 
conditions equally favourable to the action of both 
manures. It is generally considered, however, that 'if 
such conditions could be secured, the acid phosphates, 
being more readily soluble, would prove more readily 
available to the plants, and that 28 cwts. of super- 
phosphate would produce a larger increase than 20 cwts. 
of basic slag. 

From the purely practical point of view it is more 
important to determine which of the two manures will 
give the larger return for a similar expenditure. At 
the ordinary market rates, basic slag generally costs 
rather less per ton than superphosphate, and the differ- 
ence in price per unit of phosphate is still greater. 
For a given sum of money, a considerably larger quan- 



PHOSPHATIC MANURES 233 

tity of phosphoric acid can, therefore, be applied to the 
soil in the form of Thomas' phosphate, but the difference 
in the results obtained is often very small when the 
conditions are moderately well suited to the action of 
both manures. When this is not the case, the difference 
in favour of one or other may be very marked, and the 
question as to which of the two should be preferred can 
only be determined by experiment in each particular 
case. 

Application of Basic Slag. Thomas' phosphate is com- 
monly applied at the rate of from 5 to 10 cwts. per acre. 
In the opinion of many men of large experience, anything 
less than half a ton per acre is of comparatively little 
use. In experiments conducted by the author, however, 
on the application of basic slag to grass lands, it was 
invariably found that, in the end, better results were 
obtained by applying small quantities every year, than 
by applying the same total quantity at a single dressing. 
Basic slag is generally applied to grass in the back end of 
the year or as early as possible in the spring in order that 
it may become more thoroughly mixed with the soil. For 
other crops it may be applied any time before sowing the 
seed and harrowed in, but for roots better results are 
obtained by applying it to the rows at the time of drilling 
the seed than by broad -casting during the winter or early 
spring. 

It should not be mixed with acids or acid salts, or even 
with neutral salts which react with lime. For example, 
if it be mixed with sulphate of ammonia the following 
reaction takes place: 

(NH 4 ) 2 S0 4 + CaO = CaS0 4 + 2NH 3 + H 2 

Sulphate of Lime. Calcium Ammonia. Water, 
ammonia. sulphate. 

This change, it will be seen, not only causes loss of 
ammonia but also tends to destroy the basic character of 



234 SOILS AND MANURES 

the slag. It does not react with nitrate of soda, but on 
all grounds it is much better that it should not be mixed 
with that or any other substance before it is applied to 
the soil. 

Mechanical Condition of Basic Slag. The manurial 
efficiency of basic slag depends very largely upon the 
fineness. Samples which contain a comparatively small 
proportion of phosphates will often produce a better 
result than those which are richer in that respect, but 
not so finely ground. The fineness cannot be judged 
with any degree of accuracy by inspection ; it must be 
tested with a sieve. Not less than 80 per cent, should 
pass through a piece of wire cloth having 100 wires to 
the inch, i.e., 10,000 holes to the square inch. 1 

Adulteration of Basic Slag. Basic slag is very rarely 
adulterated with chalk or other non-phosphatic ingre- 
dients. If the price be fixed as it generally is in pro- 
portion of the amount of phosphate it contains, such 
admixture would be a source of loss, not of gain, to the 
seller. It has, however, occasionally been adulterated to 
a considerable extent by admixture of finely ground 
native phosphates. The presence of these substances is 
easily detected by microscopic examination. Tricalcic 
phosphate introduced into the converter along with the 
raw slag previously to the operation by which Thomas' 
phosphate is produced, cannot be detected in this way. 
It is doubtful whether phosphates so introduced would 
be regarded as adulterants. If not converted into basic 
phosphates, they would remain comparatively insoluble 
and of lower agricultural value. Purchasers of basic 
slag would therefore be well advised to obtain a guar- 
antee of the solubility of the phosphates. Many firms 
now give a guarantee that 80 per cent, pf the total phos- 

1 The standard sieve for this purpose is that known as 100 E of 
Amandus Kahl, which is of slightly larger mesh. 



PHOSPHATIC MANURES 235 

phate is soluble in dilute citric acid, and others guarantee 
the actual precentage of phosphate soluble in that reagent. 
Such phosphate is probably all basic phosphate and readily 
available to plants. 

In purchasing superphosphates, only one thing has to 
be considered the percentage of soluble phosphates. In 
purchasing basic slag, three things should be taken into 
account: The percentage of total phosphates, the solu- 
bility of the phosphate, and the fineness of the sample. 
The solubility probably depends, to some extent, upon the 
fineness, but when the former is guaranteed the latter is 
of secondary importance. 

Artificial Basic Slag. The demand for Thomas' phos- 
phate has increased so much in recent years that attempts 
have been made to produce basic phosphates independently 
of the iron industry. The process consists in fusing 
apatites and phosphorites with an ordinary slag or flux 
usually consisting of silica and lime. The product bears 
a fairly close resemblance to Thomas' phosphate and has 
been called artificial basic slag. The tricalcic phosphate 
appears to be converted into a basic form, and thus 
rendered more available to the plants. Over 90 per cent, 
of the total phosphate is soluble in ammonium citrate. In 
some varieties, e.g., the Wiborgh phosphate, potash is 
introduced in the form of felspar. 

Precipitated Phosphate. Basic slag had been in use 
for some years before it was realised that the phosphates 
contained in the spent slag might be useful for agricul- 
tural purposes. Then it was feared that the large propor- 
tion of ferrous iron, sulphides, and other substances 
known to be more or less poisonous to vegetation, might 
prove injurious to the crops, and numerous processes 
were invented to obviate this difficulty. One of the most 
successful of these was to dissolve out the phosphate 
with dilute acid, and reprecipitate it, chiefly in the form 



236 SOILS AND MANUKES 

of dicalcic phosphate, by the addition of an appropriate 
amount of lime. This product was called precipitated 
phosphate, and would probably have become a useful 
and popular manure had it not been discovered that 
better results could be obtained by simply grinding 
the slag to a fine powder and spreading it on the land 
direct. Any process of solution is not only more ex- 
pensive to carry out, but it also removes the free lime 
and destroys the basic character which is one of the most 
valuable qualities of slag. Moreover, any treatment of 
the kind is superfluous. The total amount o,f injurious 
substances in an ordinary dressing of basic slag is small 
compared with the mass of the soil, and when the slag is 
finely ground, these are rapidly oxidised. At all events 
no appreciable injury to the crops has ever been traced 
to them. 



OFTHE 

UNIVERSITY 




CHAPTEE IX 

PHOSPHO-NITKOGENOUS MANURES 

UNDER this head are included a number of products 
which contain appreciable quantities of both phosphates 
and nitrogen. They are all waste or bye-products of 
animal origin, but most of them are subjected to special 
treatment to render them suitable for manurial pur- 
poses before they are put on the market. 

They are not generally relegated to a separate class, 
but are treated as phosphatic manures or nitrogenous 
manures according to the nature of the predominant 
constituent. For example, bones containing over 45 per 
cent, of phosphates and less than 4 per cent, of nitrogen, 
are generally treated as phosphatic manures, and meat 
meals containing more than 10 per cent, of nitrogen and 
less than 5 per cent, of phosphates are dealt with under 
nitrogenous manures. It must, however, be remembered 
that, notwithstanding the great difference in the pro- 
portions of the two ingredients, nearly half the value 
of the bones is attributable to the nitrogen. Also, although 
in some samples of meat meal, etc., the proportion of 
phosphates may be negligible, in others it actually exceeds 
that of the nitrogen, and must be taken into account. It 
is therefore convenient, as well as theoretically sound, 
to treat the phospho-nitrogenous products as a separate 
group, intermediate between the phosphatic manures on 
the one hand, and the nitrogenous manures on the other. 

General Properties. It is characteristic of these pro- 
ducts that the fertilising ingredients are insoluble in 
water, and only become available to plants when the 



238 



SOILS AND MANUKES 



substance is decomposed. The change may be brought 
about by a natural process of fermentation, or as a 
result of artificial treatment. Consequently they act best 
on light open soils moderately well supplied with lime, 
and some of them are very suitable manures for this 
class of soil. Even under the most favourable conditions 
their action is relatively slow, i.e., as compared with the 
very rapid effects of sulphate of ammonia, etc. They 
are, however, valuable manures, and when they can be 
obtained at a reasonable price, are useful for many 
purposes. With the addition of potassium compounds 
they form general manures. 

BONES. 

Special interest attaches to the use of bones as manure, 
both historically and economically. They appear to have 
been, at first, employed chiefly for pastures as a means 
of restoring some of the principal ingredients removed 
from the land by grazing stock. They were afterwards 
applied to other crops, and soon became extremely 
popular. It is estimated that something like 100,000 tons 
of bones are now annually used as manure in this 
country. About one half of this quantity is collected 
at home, and the remainder is imported from abroad. 

Imports. The following tables show the quantities 
of bones for use as manure, imported into the United 
Kingdom in 1907 from various countries, and the total 
quantities in each of the last ten years : 

Tons. 

Argentina 17,491 



Belgium 
Holland . 





.... 




1,213 
2,180 


Eussia 


( 


. . . 




1,992 


Egypt 
Other countries 





. 




1,246 
6,673 



Total . 46,115 



PHOSPHO-NITROGENOUS MANURES 



239 



Year. 


Tons. 


Year. 


Tons. 


1898 


59,406 


1903 


52,996 


1899 


68,915 


1904 


35,103 


1900 


68,137 


1905 


47,346 


1901 


57,748 


1906 


42,604 


1902 


58,973 


1907 


46,115 



Composition of Bones. The ash of bones consists 
almost entirely of tricalcic phosphate, but contains also 
a small proportion of magnesia and fluorine. It can be 
removed by the action of dilute hydrochloric acid which 
dissolves out the phosphate and leaves behind the organic 
matter or combustible part of the bones. The latter 
contains two principal ingredients fat and ossein. The 
fat can be extracted by solvents or by heat. The ossein 
is an albuminoid substance, and in the dry state con- 
tains about 17 per cent, of nitrogen. On prolonged 
boiling with water it is converted into the familiar sub- 
stance known as glue or gelatine. 

The following analysis shows the average composition 
of mammalian bones in the fresh or green state : 



PER CENT. 



Water .' 
Organic matter . 

Ash 



6-7 
40-0 



[fat 
I ossein 



( P 2 5 ! 
53-3 CaO 

( Mg. F., etc. 

100-0 



6-7 
14-6 

25'4 = 4'0 nitrogen. 
22-3 = 48'7 Ca 3 (P0 4 ) 2 . 
29-2 

1-8 



100-0 



The proportions of these constituents, however, vary 
considerably in the bones of animals of different age, and 
species, and in bones taken from different parts of the 
skeleton. 



240 SOILS AND MANUEES 

A large part of the bones collected in this country have 
been cooked, and of those imported from abroad, some 
have been buried, and others had been exposed to the 
weather, often for a long time. Ordinary commercial 
bones have therefore a different composition from the 
above. They generally contain more ash and less organic 
matter, and are consequently richer in phosphates and 
poorer in nitrogen. 

Methods of Treatment. Bones which come into the 
market in the fresh state are not used directly as 
manure, but are first subjected to some process for 
extraction of the fat. The fat is of considerable value 
as tallow, and after its removal the residue is corre- 
spondingly richer both in phosphates and nitrogen. The 
presence of fat in bones makes them much more difficult 
to grind, and retards their decomposition in the soil. 

Sometimes the fat is extracted with benzine or other 
suitable solvent, which has little pr no effect on 
the ossein, but more commonly it is removed by 
steaming or boiling. After removal of the fat, the bones 
may be either ground up and sold as manure, or they 
may be subjected to a further process of steaming in 
order to extract the gelatine. The residue left after the 
further steaming crumbles very readily into a fine 
powder sometimes used as manure under the name of 
steamed bone flour, or simply steamed bones. It is, of 
course, comparatively poor in nitrogen. The proportion 
of nitrogenous matter (ossein) left in the residue de- 
pends upon the length of time they have been subjected 
to the steaming or boiling process. When it has been 
carried very far, the last traces of organic matter are 
often removed by burning, and bone ash remains. Bone 
ash has also been used as manure, but there is a Con- 
siderable demand for it for other purposes, which main- 
tains the price at a higher level than its agricultural value. 



PHOSPHO-NITBOGKENOTJS MANUEES 241 

Bones are also subjected to a process of destructive 
distillation for the purpose of making bone char, which 
is largely used in sugar refineries. In this process the 
whole of the nitrogen is volatilised, but the phosphate 
remains. The bone char is therefore fairly rich in 
phosphates, and in the spent condition is sometimes used 
as manure, but more frequently it is burned to bone ash. 

Finely powdered bones moistened with water and piled 
up in heaps readily undergo fermentation, and the nitro- 
genous matter is thus rendered more readily available. 
The product is sometimes dealt in commercially under 
the name of fermented bones. It has much the same 
composition and properties as ordinary bone meal. 

By far the most effective way of hastening the action 
of bones is to treat them with sulphuric acid. The 
product is called dissolved or vitriolated bones, and is 
of quite a different character from any of the bone pro- 
ducts previously mentioned. 

Crushed Bones. Ground or crushed bones are pre- 
pared from samples which have been altered by cooking, 
exposure, etc., or from fresh bones after removal of the 
fat. They are sometimes known as " raw bones " to 
distinguish them from those which have been vitriolated, 
or from which a considerable proportion of the nitro- 
genous matter has been removed by steaming. 

The composition, is of course, variable, but in general, 
they contain from 45 to 55 per cent, of tricalcic phos- 
phate, and from 3 to 4 per cent, of nitrogen. Both 
constituents are insoluble in water, and the action of 
bones is therefore slow. In light open soils, if not too 
dry, they are much more rapidly oxidised than in those 
of closer texture. 

The activity of bones, like that of other manures, 
depends to a large extent upon the size of the pieces, 
and they are now generally used in the form of fairly 

S.M. B 



242 SOILS AMD MANUKES 

fine powders called bone meal, bone dust, bone flour, 
etc., of which at least 90 per cent, should pass through a 
sieve having eight wires to the inch. Less finely ground 
specimens are called quarter-inch bones and half-inch 
bones when 90 per cent, will pass through sieves having 
respectively four wires and two wires to the inch. The 
more finely ground qualities are, of course, the more 
expensive, but they are distinctly the more remunerative. 
Crushed bones have been used chiefly for pastures and 
roots, but, on suitable land, are considered very good for 
hops and garden crops, especially fruit. They are applied 
at the rate of from 3 to 6 cwts. per acre, generally at the 
back end of the year. 

Steamed Bones. The term " steamed bones " is 
properly applied only to samples from which a consider- 
able part of the nitrogenous matter has been removed 
by steaming. They may be regarded as intermediate 
between ordinary crushed bones and bone ash. They 
contain from 60 to 70 per cent, of tricalcic phosphate, 
and from, 1 to 2 per cent, of nitrogen. The proportion 
of phosphates is, of course, largest in samples from 
which the largest proportion of the nitrogenous matter 
has been abstracted. They are easily ground and are 
generally obtained in the form of an impalpable powder 
which can be very thoroughly mixed with the soil, and 
is easily soluble in dilute acids. 

The powder is very light, and if sown broadcast in 
windy weather, much of it may be blown away. To 
avoid this, it is sometimes moistened with water, but a 
better plan is to mix a quantity of fine damp soil or 
sawdust with it. 

Bone Ash. This is a purely phosphatic substance 
not a phospho-nitrogenous manure. When pure it con- 
sists almost entirely of tricalcic phosphate. The ordinary 
commercial product contains from 75 to 85 per cent, of 



PHOSPHO-NITROOENOUS MANUEES 



2413 



that ingredient, but no nitrogen. It is more readily 
soluble, and has, therefore, a higher agricultural value 
than mineral phosphates containing the same percentage 
of phosphoric acid, but since the introduction of basic 
slag, it has been but little used as manure. 

Bone Char. Bone char or bone black consists practi- 
cally of bone ash plus about 10 per cent, of carbon.. 
When spent, it may be used as a phosphatic manure, 
but is generally converted into bone ash. 

Typical analyses of the various bone products men- 
tioned above are given in the following table: 

ANALYSES OF BONE PRODUCTS. 
PER CENT. 






Bone 
Meal. 


Fer- 
mented 
Bones. 


Steamed 
Bones. 


Bone Char. 


Bone Ash. 


Phosphoric acid l 


22-84 


22-32 


29-81 


36-78 


37-87 


Organic matter 2 . 


31-56 


29-37 


11-62 


10-07 (c) 


0-95 (c) 


Water 


10-37 


11-81 


8-41 





1-25 


Lime, magnesia, etc. . 


33-48 


34-64 


48-06 


52-34 


53-49 


Sand .... 


1-75 


1-86 


2-10 


0-81 


6-44 




100-00 


100-00 


100-00 


100-00 


100-00 


1 Equal to tricalcic 












phosphate . 


49-85 


48-74 


65-07 


80-29 


82-67 


2 Containing nitrogen . 


3-82 


3-53 


1-40 






Equal to ammonia . 


4-63 


4-29 


1-71 







Dissolved or Vitriolated Bones. Dissolved bones bear 
much the same relation to the raw bones from which 
they are prepared, as superphosphate bears to the raw 
native phosphates. The latter are often called mineral 
superphosphates to distinguish them from the organic, 

E2 



244 SOILS AND MANURES 

i.e., the bone superphosphates as dissolved bones have 
been occasionally called. The name " bone superphos- 
phate " has also been applied to samples of ordinary 
(mineral) superphosphate with which a quantity of raw 
bones has been mixed. Such a mixture has much to 
recommend it for certain purposes, but it is generally 
better to apply the two substances to the soil separately 
the latter preferably at the back end of the year, and 
the former in the spring. In any case such mixtures 
should not be confused with dissolved or vitriolated bones. 
They have often been sold under that name, but it is 
now illegal to do so, and they are generally described as 
dissolved bone manures or bone Compounds. 

It is sometimes argued that they are just as good as 
pure dissolved bones, but if that be true, the purchaser 
should remember that the two substances can generally 
be obtained separately at a cheaper rate. 

In some districts there is a popular idea that the 
terms dissolved bones and vitriolated bones have refer- 
ence to some difference in the treatment or composition of 
the manure, but they are really only alternative names 
for the same substance. 

The acid used in the treatment of bones is generally 
more concentrated than that employed in the manufacture 
of mineral superphosphates. As a rule only about half 
the total phosphate is rendered soluble, but the re- 
mainder is partially acted upon, and is largely converted 
into the dicalcic form. A larger quantity of acid cannot 
well be used, as the product has a tendency to become 
wet and sticky, and in that condition cannot be evenly 
distributed over the land. 

The composition of dissolved bones depends .partly 
upon that of the raw bones from which they are pre- 
pared, and partly upon the quantity of acid with which 
they have been treated. In general, they contain from 



PHOSPHO-NITROGENOUS MANURES 24o 

10 to 20 per cent, of tricalcic phosphate rendered soluble, 
a similar quantity undissolved the total phosphate varies 
from about 25 to 35 per cent. and from 2J to 3J per 
cent, of nitrogen. 

The following analysis shows the composition of a 
typical sample: 

Per cent. 

Phosphoric acid (soluble) 1 5*76 

(insoluble) 2 8*38 

Organic matter 3 and combined water .... 27*43 

Moisture ......... 12*15 

Lime, magnesia, etc 42*86 

Sand and insoluble matter . 3 '42 



100*00 

1 Equal to tricalcic phosphate rendered soluble 12'57 ) QQ.gg 

2 Equal to tricalcic phosphate undissolved . 18*29 ; 

3 Containing nitrogen . . . . . 2*40 
Equal to ammonia 2*91 

The soluble phosphate in dissolved bones is of no 
greater value than that of ordinary mineral superphos- 
phate, but the insoluble portion, i.e., insoluble in water, 
is to a large extent soluble in ammonium citrate, and 
is not only more valuable than raw mineral phosphate, 
but is probably much more readily available than the 
phosphates in untreated bones. The. nitrogenous matter 
is also partially acted upon and rendered more readily 
available. 

It appears, therefore, that dissolved bones contain the 
fertilising ingredients in varying degrees of solubility. 
A portion is soluble in water, and directly available to 
the plants, and the remainder, though insoluble in water, 
probably becomes gradually available as required. It is 
doubtless to this graduated solubility that dissolved bone 
largely owes its valuable fertilising properties. Similar 
conditions prevail in farmyard manure, and as will be 



246 SOILS AND MANUEES 

shown later can be produced by mixing, or the simul- 
taneous use of several substances, but cannot be obtained 
in any other single artificial manure. By mixing to- 
gether superphosphate, mineral phosphate, and sulphate 
of ammonia in suitable proportions, a compound manure 
could be produced containing soluble and insoluble phos- 
phates and nitrogen in the same proportions as dissolved 
bones, but as the solubilities of the ingredients would be 
very different it could not be expected to produce the same 
results. It is. therefore generally advisable to purchase 
pure dissolved bones rather than a mixture or compound 
manure at a price, perhaps, only a little lower. The 
latter may have the same composition and may contain 
a certain amount of bone, but cannot be relied upon to 
possess the same graduated solubility. 

Dissolved bone is a very valuable manure. It acts 
well on almost every class of soil, is suitable for all 
crops that require phosphatic manures, and may be 
applied either in autumn or spring. It is too expensive 
to recommend for the ordinary purposes of the farm, 
but has been largely employed for garden crops, for which 
it appears to be particularly suitable. 

MEAT MEALS. 

In the preparation of meat extracts, the fat and bone 
are separated, and the purely fleshy part of the tissue is 
subjected to a process of prolonged boiling. When all 
the soluble matter has been so extracted, the residue is 
dried, ground to a fine powder, and sold as manure under 
the name of flesh or meat meal, or sometimes meat 
guano. Considerable quantities are produced at home, 
and more is imported from abroad. The best known 
variety comes from Freybentos, in Uruguay. 

Similar products are now made from slaughter -house 



PHOSPJBO-NITROGENOUS MANUEES 247 

refuse, and from the carcasses of diseased animals of 
all kinds. The fat, which is used as tallow, and 
usually also a quantity of gelatine are removed by steam- 
ing, and the residues are dried, ground up, and sold as 
manure. The last-mentioned product is made chiefly in 
Germany. A certain amount of bone is always included, 
and it is therefore often called phosphatic meat guano 
to distinguish it from the almost purely nitrogenous 
variety obtained in the preparation of meat extract and 
some other processes in which it is a bye-product. 

Nitrogenous samples contain from 10 to 13 per cent, 
of nitrogen, and from 1 to 3 per cent, of phosphate. 
The composition of phosphatic samples is more variable, 
the proportions of bone and moisture being the chief 
determining factors. They may contain from 10 to 20 
per cent, of phosphates, and from 4 to 8 per cent, of 
nitrogen, but all kinds of intermediate samples are met 
with, from the most nitrogenous to the most phosphatic. 
Even the last, it will be seen, is mainly a nitrogenous 
manure. The phosphates and nitrogen are both insoluble, 
but as the manure is usually in the form of a very fine 
powder, and on open soils is fairly rapidly oxidised, they 
soon become available to the plants. 

FISH GUANO. 

Fish guanos do not consist of the excrement of fishes, 
but of dried fish offal, and are more properly described 
as fish meals. They are produced at the fish curing 
stations in this country, and are imported from abroad, 
chiefly from Norway and America, in large quantities. 
They are also made from surplus and decayed whole 
fish, the carcasses of whales, etc., and are often called 
herring guano, whale guano, etc., according to the kind 
of animal from which they are chiefly prepared. As in 



248 SOILS AND MANURES 

the case of meat meals, to which they are closely ana- 
logous, the composition depends largely upon the amount 
of bony matter they contain. 

The Norwegian product is made chiefly from cod, and 
contains from 7 to 10 per cent, of nitrogen, and from 
12 to 14 per cent, of phosphates. The American fish 
guano is made from herrings ; it contains about the same 
proportion of nitrogen, but is generally poorer in phos- 
phates. 

The fertilising ingredients are insoluble and only be- 
come available as the substance is oxidised. The large 
amount of oil naturally present in the fish hinders decom- 
position, and retards the action of the manure. The bulk 
of the fatty matter is, however, generally extracted in the 
course of manufacture, but purchasers of fish manure 
should obtain a guarantee that it contains not more than 
3 per cent, of oil. The percentages of phosphates and 
nitrogen should, of course, also be guaranteed. Like meat 
meals they are mainly nitrogenous manures, and give their 
best results on the lighter class of soils. 



CHAPTEE X 

NITROGENOUS MANURES 

THE nitrogenous manures are in many respects the 
most important of the special manures. They are, 
perhaps, not so universally popular as the phosphatic 
manures, but the potency of their effects, their relatively 
high price, and the fact that if ignorantly or carelessly 
used, they may do harm rather than good, all combine 
to give them a peculiar prominence. They may be divided 
into organic substances and salts ; of the latter, sulphate of 
ammonia and nitrate of soda are the most important. 

ORGANIC NITROGENOUS MANURES. 

This division of the subject must be regarded as 
practically a continuation of the chapter on phospho- 
nitrogenous manures. The distinction between the two 
groups, it has been said, is rather an arbitrary one. Fish 
meals and meat meals are mainly nitrogenous manures, 
and are generally treated as such notwithstanding that, 
in some cases, they contain considerable quantities of 
phosphates. 

The products now to be dealt with are those in which 
phosphates are absent altogether, or are present only in 
negligible quantities. They are all waste or bye-pro- 
ducts of animal origin. The nitrogen is present in the 
form of insoluble compounds, and only becomes avail- 
able to plants as the substance becomes oxidised and 
decomposed. They are best suited for open soils, and 



250 SOILS AND MANURES 

are slower in action than ammonium salts and nitrates. 
Some of them, indeed, act so slowly as to be unsuitable 
for direct application to the soil. They can be rendered 
more quickly available by subjecting them to a prelim- 
inary process of fermentation or composting. Treatment 
with sulphuric acid is still more effective, and they are 
chiefly used as a source of nitrogen in the manufacture 
of the so-called compound artificial manures. The fol- 
lowing are among the most important. 

Ground Leather. Ordinary leather is prepared from 
the hides of animals, which, in the dry state, contain 
about 16 per cent, of nitrogen. The hides are treated 
with tannin, and impregnated with grease in order to 
preserve the nitrogenous matter from decomposition. 
This, of course, greatly reduces the manurial value, which 
depends, in large measure, upon the rate at which the 
substance decomposes. The scrap leather which is col- 
lected, is generally found to have lost a considerable 
proportion of the grease, and when dried and ground to 
powder is occasionally used as manure. It contains 
about 5 per cent, of nitrogen, but its action is so slow 
that it cannot be economically used directly in the un- 
treated condition. Terrified leather is the name given 
to samples from which the fat has been extracted by the 
action of superheated steam. It contains a somewhat 
larger proportion of nitrogen, and decomposes more 
readily than the untreated samples, but even then it 
cannot be recommended for direct use as manure. 

Hoof and Horn Meals. Hoofs and horn, like hides, 
flesh, etc., belong to the class of albuminoid substances, 
and have a similar composition, but are much drier and 
therefore, in the natural state, contain a larger propor- 
tion of nitrogen. They decompose, however, more slowly 
and are less suitable as manure, even when very finely 
ground. They are occasionally mixed with meat meals 



NITEOGENOUS MANUKES 251 

and other scrap manures, but as they are of lower agri- 
cultural value, such mixture may be regarded as of the 
nature of adulteration. The action can be hastened by 
steaming, fermenting, etc., but the difference produced 
by such treatment is not sufficient to make them suit- 
able for ordinary agricultural purposes. The proportion 
of nitrogen varies in commercial samples from about 
7 to 15 per cent. The average is about 12 per cent. 

Hair and Feathers. Waste hair and feathers are also 
sometimes used as manure. They have much the same 
composition and properties as horn. 

Woollen Rags and Shoddy. Wool may be regarded 
as only a slightly differentiated kind of hair. It has 
almost exactly the same chemical composition and very 
similar properties. In the pure dry state, after removal 
of the fat, it contains over 16 per cent, of nitrogen. 
Ordinary mixed woollen rags, however, are not so rich 
because a considerable quantity of cotton is now com- 
monly woven into many of the so-called "woollen" 
fabrics. 

Shoddy is the name given to the waste from the woollen 
factories. It also contains varying quantities of cotton, 
besides oil, dust, and other impurities, and is of very 
uncertain composition. The proportion of nitrogen varies 
from about 4 to 12 per cent., but the average is about 
6 or 7 per cent. Woollen rags and waste have been used 
as a nitrogenous manure for root and other crops, but 
their action is too slow to be of much benefit. They are 
considered more suitable for hops and garden crops, but 
should always be composted before use. 

Dried Blood. Dried blood is a substance of very 
different properties from any of those mentioned above. 
It is generally richer in nitrogen and more active than 
the best nitrogenous meat meals, and may be regarded as 
only slightly inferior to sulphate of ammonia. For 



252 SOILS AND MANUEES 

some purposes it possesses certain advantages, and as it 
is much cheaper, both per ton and per unit of nitrogen, 
it has much to recommend it. The blood is collected in 
slaughter-houses, and gently dried by steam heat, when 
it is obtained in the form of dark brittle flakes, which are 
easily reduced to impalpable powder. In the powdered 
condition in which it is sold it is of a dark ruddy brown 
colour, and possesses a characteristic, rather offensive 
odour. It can be kept indefinitely without decomposing, 
but if allowed to become damp is liable to ferment. It 
is easily distributed, but is liable to be blown away, and 
should therefore be mixed with some damp soil before 
it is spread on the land. It is sold in -two qualities des- 
cribed as " low dried " and " high dried " respectively. 
The former contains about 14 per cent, of nitrogen, does 
not keep so well, but is the more active and cheaper 
form. The high dried quality contains a smaller pro- 
portion of water, an'd therefore more nitrogen usually 
about 16 per cent. keeps better, but is not quite so 
readily available, and is considerably more expensive, i.e., 
it costs more per unit of nitrogen. This is due to the 
fact that as the drying proceeds it becomes more and 
more difficult to expel the remaining water without car- 
bonising the organic matter. If over -heated, it acquires 
a brown or singed appearance, loss of nitrogen results, 
and the remainder is probably r^nderod less readily 
available. 

As the nitrogen in dried blood is not directly available, 
it should be applied fairly early in the spring. For the 
same reason it is best adapted for use on light soils, for 
which it is very suitable as there is no danger of loss in 
wet weather. Under favourable conditions it acts very 
quickly, i.e., as compared with hair, horn, etc., but its 
action is slow compared with that of nitrate of soda. It 
is not so suitable as the last mentioned substance for 



NITROGENOUS MANUEES 253 

grass and cereals, but gives very good results with 
turnips, potatoes, fruit, and garden crops. It repro- 
duces some of the conditions which obtain in farmyard 
manure, and favours a slow steady growth. 

SULPHATE 01 AMMONIA. 

Ammonia is produced by the decay of nitrogenous 
organic matter, or more rapidly by the destructive dis- 
tillation of the same. It is therefore a bye-product in 
the manufacture of illuminating gas, shale oils, bone 
char, etc., which are commercially obtained by the de- 
structive distillation of coal, bituminous shales, bones, 
horn, etc. When neutralised with sulphuric acid, it yields 
sulphate of ammonia thus : 

2NH 3 + H 2 S0 4 = (NH 4 ) 2 S0 4 

Ammonia. Sulphuric Sulphate of 
acid. ammonia. 

Production. Ordinary coal contains from ^ to 1 per 
cent, of nitrogen. When it is burned in open fires the 
carbon, hydrogen and sulphur are oxidised, and most 
of the nitrogen is liberated in the free state. But when 
it is heated in closed retorts, as in the manufacture of 
coal gas, oxidation cannot take place, and a large pro- 
portion of the nitrogen goes off in the form of ammonia. 
The gases are passed through a cooling apparatus in 
which the aqueous vapour condenses and retains the 
ammonia and other soluble substances. The liquid col- 
lects in a tank placed to receive it and is called the 
" ammoniacal liquor." A gallon of this product contains 
about three ounces of ammonia, chiefly in the form of 
hydrate and carbonate, and smaller quantities of chloride, 
sulphide, sulphate, thiosulphate and sulphocyanide. 

In order to separate the ammonia from the impurities, 



254 



SOILS AND MANURES 




the liquor is saturated with 
lime and distilled ; the 
volatile ammonia gas passes 
over, is neutralised with sul- 
phuric acid, and the salt thus 
formed is crystallised from 
saturated solutions. A typical 
form of plant used in gas- 
works for this purpose is 
shown in the illustration l 
(Fig. 31). The product ob- 
tained in this way is practi- 
cally free from all impurities 
except those contained in the 
sulphuric acid or picked up 
incidentally. It always con- 
tains a certain amount of 
moisture, but in well-made 
samples the total solid im- 
purity should not exceed 2 or 
3 per cent. 

The production of sulphate 
of ammonia in other indus- 
tries is carried on in a very 
similar manner. About 65 
per cent, of the total output 
comes from gasworks, 20 per 
cent, from shale-works, 10 per 
cent, from iron-works, and the 
remaining 5 per cent, from 
various carbonising works. 
More than 300,000 tons is 

1 Copy supplied by Messrs. R. 
and J. Dempster, Ltd., Manchester. 



NITROGENOUS MANURES 255 

now produced annually in this country alone 1 ; a large part 
of it is exported, but about 87,000 tons is consumed at 
home, of which from 80 to 90 per cent, is used in agricul- 
ture. 

Composition and General Properties. According to the 
chemical formula given above, sulphate of ammonia con- 
tains 25'76 per cent, of ammonia, which is equivalent to 
21 '21 per cent of nitrogen. When pure, it presents the 
appearance of a colourless, odourless, crystalline salt. 
The commercial product should contain not less than 
24*5 per cent, of ammonia, which is equal to 95 per 
cent, pure sulphate of ammonia. The impurity generally 
consists of sand, common salt and moisture. A small 
amount of tarry matter is occasionally present, and 
imparts a brown or blue colour and characteristic odour, 
but does not affect the manurial value. Sulphides and 
sulpho-cyanides are rarely found in samples which have 
been prepared by distillation. The former can be detected 
by the odour of sulphuretted hydrogen it resembles that 
of rotten eggs which is given off when sulphuric or 
hydrochloric acid is added to the sample. The detection 
of sulpho-cyanides is equally simple. A few grains of 
the sulphate of ammonia are dissolved in a wine-glassful 
of water, and a drop of ferric chloride is added to the 
solution. If sulpho-cyanide be present a dark red colour 
is produced, but if absent no change will be observed. 

Adulteration. The high price which sulphate of 
ammonia commands, offers a certain temptation to adul- 
terate it. The substances which have been most com- 
monly employed for this purpose are gypsum, common 
salt, and sulphate of soda. Two simple tests may be 
applied as follows: (1) Anything that remains undis- 
solved when the substance is stirred up with water is 

1 Frank, Chemical Trade Journal, 1908. 



256 SOILS AND MANUKES 

impurity ; (2) Any non-volatile substance which remains 
when the substance is heated to redness is also impurity. 
If the proportion of insoluble or non-volatile matter 
appears to be unusually large, it is to be regarded with 
suspicion. The only reliable test, however, is the quan- 
titative determination of the ammonia. This test can 
only be carried out in a properly equipped laboratory, 
but it should never be neglected, as it is an insurance, 
not only against intentional fraud, but also against the 
accidental presence of any undue quantity of impurity. 

Manurial value. The manurial effects of sulphate of 
ammonia depend upon its concentrated character, solu- 
bility, and capacity for nitrification. 

With the exception of ammonium chloride, which is 
rarely or never used, and the recently introduced nitrolim, 
it is the most highly concentrated of all nitrogenous 
manures, i.e., it contains the largest proportion of nitro- 
gen. Other substances, e.g., dried blood, meat meals, 
etc., may contain as much as 16 per cent, of nitrogen 
they usually contain less but sulphate of ammonia con- 
tains about 20 per cent. Small quantities may therefore 
be expected to produce relatively large effects, and ex- 
perience shows that such is the case. 

When applied to the soil it rapidly undergoes nitrifi- 
cation. The ammonia is thereby converted into nitric 
acid, which is the most suitable form of nitrogen for the 
nourishment of plants. The presence of a certain amount 
of lime in the soil is, therefore, essential for its proper 
action. The lime is required both to effect the liberation 
of the ammonia from the sulphuric acid, and to maintain 
the conditions in the soil under which alone the nitrifying 
organisms can act. 

On soils which are deficient in lime, sulphate of 
ammonia is said to be positively injurious. On the other 
hand, if the proportion of lime is excessive, as in the case 



NITEOGENOUS MANUBES 257 

of chalky soils, a certain amount of loss may occur 
through the evaporation of ammonia before it is con- 
verted into nitric acid. In general the soil exhibits 
great power of absorbing and retaining ammonia, and 
it has been shown by experiment that nitrogen in that 
form is suitable for the nourishment of plants. Under 
ordinary conditions, however, nitrification takes place 
so rapidly that probably very little ammonia is absorbed 
by plants and still less is lost by evaporation^ 

Sulphate of ammonia is completely and readily soluble 
in water. It is therefore very rapid in its action, i.e., 
as compared with organic products, such as meat meals, 
etc., in which the nitrogen is insoluble and passes through 
a series of changes ammonia is probably one of the 
stages (p. 153) before it can be absorbed by plants. 
Sulphate of ammonia is not, however, so readily soluble 
as nitrate of soda, and as some time is required for 
nitrification, its action is not so rapid as that of the last- 
mentioned substance. It should therefore be applied 
rather earlier, and is perhaps to be preferred for the 
lighter class of soil as it is less liable to loss by drainage. 
It acts equally well, however, on heavy land. 

Application. Sulphate of ammonia is largely employed 
for grass and cereals, and generally gives a very good 
return. It is applied in the spring as a top dressing 
to the growing crop. When used for spring corn it is 
sometimes spread and harrowed in with the seed, but 
this practice cannot be recommended. On light land 
it involves a certain risk of loss by drainage. On stiff 
retentive land the risk is probably small, but as there is 
no great practical advantage, it is better to avoid any 
risk, by waiting until the crop is up before applying this 
manure. For root crops, it is most advantageously placed 
in the drills along with the phosphates or other manure. 
At that time of year, this can be done with tolerable 

S.M. S 



258 SOILS AND MANTJKES 

safety. It may be sown broadcast without much risk 
of injuring the plants by contact with the leaves. It is 
a very suitable form of nitrogenous manure for turnips 
and potatoes. Owing to its relatively slow action it is, 
perhaps, more suitable than nitrate of soda for these 
crops, but not for mangolds. 

It is applied at the rate of from 1 to 2 cwts. per acre, 
but the smaller quantity will generally be found sufficient 
for ordinary purposes. 

Sulphate of ammonia, it has been said, may be safely 
mixed with acid manures, such as superphosphate, dis- 
solved bones, etc., but must not be mixed with lime, 
manures, such as basic slag, which contain lime, or 
alkaline substances like wood ashes, etc. It reacts with 
these substances and gives off ammonia. 

Other Ammonium Salts. Chloride or muriate of 
ammonia, NH 4 C1, has been used as manure, but not 
often, because it costs more per unit of ammonia and is 
no better than, if as good as, the sulphate. It is more 
concentrated contains 31*78 per cent, of ammonia and 
is more readily soluble in water. Chlorides have an in- 
jurious effect on many crops, but the small quantity in 
an ordinary dressing of the ammonium salt is not notice- 
ably harmful. 

Other salts, such as ammonium nitrate, ammonium 
phosphate, etc., though theoretically very desirable, are 
never used as manures. The fertilising ingredients they 
contain can be obtained more cheaply in other forms. 

Soot. Soot, the condensed smoke of coal fires, consists 
mainly of carbon in a state of very fine division. It con- 
tains, however, a certain amount of sulphate of ammonia, 
and for manurial purposes may be regarded as a very 
impure form of that substance. The proportion of 
ammonia is naturally very variable, but usually amounts 
to between 4 and 5 per cent., which is equal to about 



NITKOGENOUS MANUEES 259 

16 or 20 per cent, of sulphate of ammonia. One cwt. 
of the latter should therefore have the same manurial 
value as 5 or 6 cwts. of soot. 

Some farmers consider that it has a valuable mechani- 
cal effect on clay soils, but no mention is made of 
its having any effect good or bad on light soils. Soot 
is very largely used in gardens, but seems to be appre- 
ciated as a preventive against the ravages of slugs and 
insects, even more than for its manurial effects. 

NITRATE OF SODA. 

Occurrence. Immense natural deposits of nitrate of 
soda or Chili saltpetre, as it is sometimes called, have 
been discovered in the arid rainless district on the borders 
of Chili and Peru on the west coast of South America. 
The deposits are found always near the coast at a height 
of three or four thousand feet above sea level. It' 
seems plain that they must have been formed by the 
nitrification of organic matter which would be promoted 
by the presence of sodium carbonate and other alkaline 
salts and basic substances with which the soil is impreg- 
nated. The process of nitrification has already been 
described. In this country the nitrates do not accumu- 
late in the soil but are washed out by the excess of rain 
water which passes into the drains. In the desert region 
in which these deposits are found, the climate is ex- 
tremely dry ; the rainfall is never sufficient to produce 
any flow of drainage, and the nitrates are not washed 
out of the soil but only carried down a little below the 
surface. 

It is more difficult to account for the enormous quantity 
of organic matter from which it is supposed the nitrate 
has been produced. Two possible sources have been 
suggested, viz., guanos, deposits of which are found 

s2 



260 SOILS AND MANUEES 

in considerable quantity in that part of the world, 
and accumulations of seaweed. The first of these two 
hypotheses seems to be untenable. Guano deposits are 
often found to have undergone change, but it is always 
the nitrogenous matter which disappears and the phos- 
phates that are left behind. It is difficult to conceive any 
kind of change by which the phosphates would be re- 
moved and the nitrogenous matter left. If the deposits 
had been formed by the nitrification of guano, phosphates 
would almost certainly be found mixed with the nitrates 
or near them, which is not the case. If the original 
organic matter consisted of seaweed, it is clear that the 
land must have undergone great elevation since it accu- 
mulated. The geological evidence indicates that such ele- 
vation actually has taken place even within comparatively 
recent times, and though there is no certainty about the 
matter, the seaweed theory is generally regarded as much 
the more probable. It is, in fact, the most probable ex- 
planation hitherto suggested. 

Extraction. The crude product, which is locally known 
as caliche, is, of course, very impure. It is not only 
mixed with sand and earthy matters, but contains also 
a considerable proportion of the chlorides and sulphates 
of sodium, magnesium and calcium, and smaller quanti- 
ties of iodine, boracic acid, etc., all of which are, to a 
large extent, eliminated in the process of refining, which 
is carried out on the spot. The proportion of nitrate of 
soda varies from about 25 to 50 per cent., the caliche 
being much richer in some districts than in others, but 
the average is about 35 per cent. 

The caliche is of varying thickness from a few inches 
to several feet and lies at some depth below the sur- 
face. The covering crust consists of two distincf layers, 
the upper, called costra, being a mixture of sand and 
gypsum, and the lower, called congelo, a conglomerate 



NITROGENOUS MANURES 



261 



of gravel and clay. The whole is blasted out by charges 
of dynamite placed underneath the caliche, which is after- 
wards separated mechanically from the fragments of 
the covering layers. It is then lixiviated with water 
to separate the earthy and insoluble matters, and the 
nitrate of soda is crystallised out from saturated solutions 
and dried in the sun. Iodine and other salts are recovered 




from the mother liquors in considerable quantities. A 
general view of the nitrate and iodine plant is shown in 
the illustration 1 (Fig. 32). The occurrence of iodine along 
with the nitrate of soda is regarded as confirming the view 
that the organic matter from which the latter originated 
consisted of seaweed. 

The nitrate fields are very extensive. Upwards of 

1 From "The World's Work," March, 1909. 



262 



SOILS AND MANUBES 



twenty million tons have already been excavated, and 
according to a report made to the Chilian government 
last year, it is estimated that there are some 220 million 
tons still left. At the present rate of production, or even 
allowing for the probable increase, it will take more than 
100 years to exhaust the supply. 

The first shipment of nitrate of soda, which took place 
from the port of Iquique in 1830, amounted to only 
800 tons. The discovery of the nitrate fields appears to 
have been made some years before that date. Ten years 
later, in 1840, the annual rate of export had risen to 
over 10,000 tons, in 1890 it had increased to over a 
million tons, and in 1907 the total quantity exported 
amounted to 1,660,000 tons. 

The following table shows the quantities of nitrate of 
soda imported into the United Kingdom during the last 
ten years : 



Year. 


Tons. 


Year. 


Tons. 


1898 


180,327 


1903 


116,715 


1899 


140,851 


1904 


120,526 


1900 


141.155 


1905 


104.436 


1901 


107,108 


1906 


108,486 


1902 


114,952 


1907 


113,894 



Composition and General Properties. Pure nitrate of 
soda contains 16*47 per cent, of nitrogen, equal to 20*0 
per cent, of ammonia, and is a colourless crystalline 
salt. The commercial product usually contains about 
95 per cent, pure nitrate of soda, which is equal to 
15*65 per cent, of nitrogen. The bnlance consists mainly 
of water, which usually forms 2 or 3 per cent., and 
chlorine, equal to from ^ to 1 per cent, of sodium 
chloride. Small quantities of sulphates, magnesia, lime, 



NITEOGENOUS MANURES 263 

and traces of iodates and some other salts are also 
generally present. Perchlorates are an impurity of much 
greater importance, as they are injurious to plants. 
These compounds have been found in samples of nitrate 
of soda in quantities equal to between 6 and 7 per cent, 
of sodium perchlorate. Their presence was originally dis- 
covered in the course of investigations into the cause of 
injuries done to crops by the application of nitrate of 
soda. Anything over 1 per cent, of sodium perchlorate 
is likely to prove harmful. Happily, a larger proportion 
than this is not of very frequent occurrence. 

Adulteration. Any tendency to fraudulent adulteration 
of nitrate of soda is largely held in check by the fact that 
so many samples are submitted to analysis the only 
means by which its purity or quality can be determined. 
In view of the high price which the substance commands, 
it is very important that this should be done, in order to 
make sure that it is neither deliberately adulterated nor 
contains any undue quantity of accidental impurity. 
There are no simple tests, beyond the fact that the sub- 
stance should be practically completely soluble in water. 
Chlorides can be detected by the addition of a drop of 
silver nitrate, and sulphates by the addition of barium 
chloride to a solution of the nitrate, but as both of these 
are commonly present in good commercial samples, such 
qualitative tests are of very little value. 

Manurial Value. The manurial effects of nitrate of 
soda may be ascribed to the fact that it is a highly con- 
centrated nitrogenous manure, contains the nitrogen in 
the form most suitable for the nourishment of plants, 
and is extremely easily soluble in water. The solubility 
cannot be too strongly insisted upon. It is, in fact, the 
key to the character of the substance. When nitrate of 
soda is applied to a growing crop its effects are generally 
manifest within a very short time, even when no rain 



264 SOILS AND MANUKES 

has fallen. The substance itself is slightly deliquescent. 
It soon absorbs moisture from the air, and the evening 
dews are sufficient to dissolve it and cause it to sink into 
the soil, whence it may be taken up by the plants. It 
is therefore particularly well suited for dry climates and 
for late dressings. 

For the same reason, i.e., because of its extreme 
solubility, it is not so suitable for very light soils. It 
acts just as well on these as on stiff land, but it is more 
liable to be washed out into the drainage by heavy rains. 
The liability to loss is perhaps not very great, even on 
moderately light soils, if it is applied as a top dressing 
to the growing crop, but if applied earlier in the year and 
harrowed in with the seed, as is sometimes done, the risk 
of loss is considerable and altogether unnecessary. So 
important is this aspect of the matter that some farmers 
have adopted the plan of dividing the total quantity to 
be applied into two or even three portions, and spreading 
them separately at intervals of ten days or a fortnight. 
This plan has its disadvantages, but is on right lines. It 
is calculated to produce the best results that can be 
obtained by the use of a given quantity of the manure. 

Application. Nitrate of soda is distinctly the most 
efficacious nitrogenous manure for grass and cereals. It is 
applied, as has just been said, as a top dressing to the grow- 
ing crop. From 1 to 2 cwts. per acre is generally allowed, 
but even smaller quantities are often highly beneficial. 
Some difficulty may be experienced in spreading a 
small quantity uniformly over a large area, and if it 
be proposed to apply it by instalments, the difficulty will 
be all the greater. It is, however, easy to swell the bulk 
by mixing the manure with a quantity of. fine soil, 
sand, sawdust, or other similar substance. 

If used for turnips and potatoes, it may be put into 
the drills along with the other manures without much 



NITROGENOUS MANURES 265 

risk of loss, except perhaps on the most open soils. 
Many experienced farmers prefer to apply it to root 
crops also as a top dressing. It should not be sown 
carelessly broadcast, because any particles which may 
remain in contact with the leaves burn holes in them 
and injure the plants. It must be remembered, how- 
ever, that the root crops are not. largely dependent 
upon nitrogenous manures, especially if farmyard manure 
has been applied to them. In any case, the writer is of 
opinion that sulphate of ammonia, or some of the slower 
acting forms of nitrogenous manure, are, in general, 
more suitable for these crops with the exception of 
mangolds. 

It has been previously pointed out that nitrate of 
soda should not be mixed with superphosphates, dis- 
solved bones, or other acid manures, before application 
to the soil. It may be safely mixed with basic slag, 
potash salts, and sodium chloride if desired, but the 
practice is not recommended. It is of no advantage to 
do so, and nitrate of soda should be applied much later 
in the season than these substances. There is no reason 
why nitrate of soda should not be applied to soils that 
have been previously treated with superphosphate or any 
other kind of manure, except perhaps, farmyard manure. 
Large quantities of the last mentioned substance in the 
fresh condition are apt to cause denitrification. 

Comparison of Nitrate of Soda and Sulphate of 
Ammonia. It is impossible to make any general state- 
ment with regard to the relative merits of nitrate of soda 
and sulphate of ammonia. All the circumstances re- 
lating to the soil, crop, climate and season, arid the 
respective prices of the two manures at the time must 
be taken into consideration in each case. Sulphate of 
ammonia contains a larger proportion of nitrogen, and 
may therefore be expected, under conditions favourable 



266 SOILS AND MANURES 

to its action, to give a larger return than an equal 
weight of nitrate of soda. For the same quantity of 
nitrogen there is often very little to choose between them, 
but nitrate of soda is the more soluble, acts more quickly, 
and for grass and cereal crops at least, the advantage, 
if any, will generally be found to lie with it. For 
turnips and potatoes, on very light soils and in wet 
climates, sulphate of ammonia may prove superior. The 
prices of both manures fluctuate so much from time to 
time that it is difficult to compare them on the basis of 
equal money values. Sulphate of ammonia generally 
costs rather more than nitrate of soda at present the 
difference is about sixpence per unit of nitrogen but it 
should be kept in view that 1^ cwt. of the former contains 
approximately the same quantity of nitrogen as 2 cwts. of 
the latter. 

Exhaustive Effects. Nitrogenous manures, and especi- 
ally nitrate of soda, are sometimes said to have an ex- 
haustive effect on the soil. A state of exhaustion, it has 
been shown, is due to removal of available plant foods 
which are abstracted by the growing crops. It is obvious 
therefore that inasmuch as nitrate of soda stimulates the 
growth of the crops it must tend to produce such effects. 
It does not follow, however, that it is inadvisable to use 
nitrate of soda or other nitrogenous manures, but they 
must be used with discretion. The available phosphoric 
acid and other plant foods which are abstracted in conse- 
quence of the action of the nitrogenous manures, must 
be replaced. In other words, phosphatic manures, etc., 
must be used in conjunction with nitrate of soda. It is 
only when used alone and for several years in succession 
that exhaustive effects are produced by the action of 
nitrogenous manures, and any such effects are not usually 
apparent until the application is discontinued. 

On the grass land at Kothamsted, plot 17, which has 



NITROGENOUS MANURES 267 

been treated with nitrate of soda every year for forty- 
seven years in succession, has produced an average crop 
of 35 cwts. of hay annually. Plot 3, which has been con- 
tinuously unmanured, has produced an average crop of 
only 22 cwts. annually. During the last ten years of 
the period, the average yield of the unmanured plot was 
16 cwts., and that of the nitrate of soda plot was 31 cwts., 
or nearly double that of the former. In the year 1905 
the yield from the unmanured plot was 19 cwts., and 
from the nitrate of soda plot over 39 cwts., or more 
than double that of the former. It will thus be seen 
that even when applied alone for forty-seven years in 
succession, the exhaustive effects of the nitrate of soda 
are not apparent so long as the application is continued, 
while there has been a marked diminution in the produce 
of the unmanured plot. 

What would happen if the application of the nitrate 
of soda were now discontinued is difficult to tell. In an 
experiment conducted by the writer, two plots were 
manured with nitrate of soda at the rate of 1 cwt. and 
2 cwts. per acre respectively, for four years in suc- 
cession, and then left unmanured for three years. Each 
year that the manure was applied, a considerable increase 
in the crop was obtained, but when the application was 
discontinued, both plots gave a smaller yield than the 
continuously unmanured plot, showing that a certain 
exhaustion had been produced. In the second year the 
1 cwt. plot had recovered, and in the third year both plots 
again gave larger crops than the unmanured plot. When 
the nitrate of soda was applied in conjunction with 
superphosphate, there was no exhaustion ; both plots 
gave a larger yield than the unmanured plot throughout 
the whole period of the experiment, as is shown in the 
table on p. 268. 

It has been pointed out that under like conditions other 



268 



SOILS AND MANUKES 



nitrogenous manures produce similar exhaustive effects 
in proportion to the rapidity and potency of their action. 
In fact, any special manure containing only one fer- 
tilising ingredient, e.g., superphosphate, kainite, when 
applied alone, must be more or less exhaustive if it has 
any effect at all. If an increase of the crop is produced 
by the action of such a manure, a larger amount of 
other plant foods will be abstracted from the soil, and 
the residue will be proportionally diminished. This fact 
has been demonstrated by experiment. 

RESIDUAL (EXHAUSTIVE) EFFECTS OF NITRATE OF SODA. YIELD OF 
HAY IN EACH OF THREE YEARS FROM PLOTS WHICH HAD BEEN 
MANURED WITH NITRATE OF SODA DURING THE FOUR PREVIOUS 
YEARS. 

PER ACRE. 



Year. 


No manure. 


Nitrate of soda alone. 


Nitrate of soda and 
superphosphate. 






1 c\vt. 


2 cwt. 


1 cwt. 


2 cwt. 




Cwts. 


Cwts. 


Cwts. 


Cwts. 


Cwts. 


1897 


26-3 


25-6 


22-3 


28-3 


25-6 


1898 


21-2 


22-2 


20-8 


22-8 


21-4 


1899 


17-3 


24-3 


21-0 


22-1 


20-5 



Nitrate of soda is not a " lasting " manure. It comes 
into action very rapidly, and if any escape absorption 
by the plants it will not remain in the soil, but will .be 
washed out in the drainage water. Still, a larger pro- 
portion of the nitrogen in nitrate of soda is recovered in 
the crops than in the case of any other manure, and it is all 
recovered in the first year. In some manures, a smaller 
proportion of the whole is ultimately recovered, and the 
process occupies some five or ten or twenty years or 
more. On grass land, the effects of nitrate of soda are 
not so transient as is sometimes supposed. The effects of 



NITROGENOUS MANURES 269 

a dressing of this manure can sometimes be traced for 
several years after it has been applied. This is probably 
due to the accumulation of humus, which results from 
the action of the manure. 

There may be some difference between a lasting manure 
and one which produces lasting effects. In general, how- 
ever, lasting effects are produced only by manures 
which come slowly into action. This, of course, is not 
an advantage but quite the reverse. The slower the action 
of the manure the longer it will last, but the less valu- 
able it will be. A manure which would last for ever 
would be of no value at all, i.e., it would not be a manure. 

Other Nitrates. There is every reason to believe that 
the nitrates of all non-poisonous bases would be equally 
suitable for manurial purposes, but practically none of 
them, except nitrate of soda, are ever used. They are 
all more expensive. 

Potassium nitrate contains two constituents of manurial 
value nitric acid and potash and where the latter sub- 
stance is required, would, for this reason, doubtless give 
a better result than an equivalent quantity of nitrate of 
soda. Many of the prescriptions for garden manures 
include a quantity of potassium nitrate, but apparently, 
the constituents are introduced in that form merely to 
simplify the recipes. The two ingredients can be ob- 
tained more cheaply separately nitric acid in the form 
of nitrate of soda, and potash in the form of sulphate of 
potash, or other similar salt. The difference in cost of 
transport is more than made up by the difference in 
price. 

Nitrate of Lime. Calcium nitrate is formed by the nitri- 
fication of organic matter in soils and composts. It could 
easily be prepared from nitrate of soda, but it would be of 
little advantage, for manurial purposes, to do so. The 
change probably takes place naturally, in the soil, and the 



270 SOILS AND MANUEES 

presence of the sodium apparently does no harm. Quite 
recently, however, a method has been devised by which 
it can be prepared directly from the air on a commercial 
scale. It is obtained by the Birkland and Eyde pro- 
cess, in which the nitrogen of the air is oxidised to nitric 
oxide by means of an electric arc furnace, i.e., in much 
the same way as nitric acid is formed naturally in the air. 
The nitric oxide is easily oxidised 'to nitrous and nitric 
acids, and the products are absorbed by powdered slaked 
lime, forming nitrate of lime. The commercial product 
contains about 13 per cent, of nitrogen which is equal to 
76'5 per cent, of calcium nitrate. About a fourth of the 
total lime remains unchanged. As a manure, it does not 
tend to decalcify the soil; on the contrary it slightly 
increases the proportion of available lime. It might, there- 
fore, be expected to produce a rather better effect than 
equivalent quantities of other nitrogenous manures, but 
reports on this point are conflicting. It is a highly deli- 
quescent substance and must be kept free from damp. 
The very fine state of division in which it is sold makes it 
difficult to handle. It cannot be sown broadcast as it 
injures the hands and eyes of the workmen, and if dis- 
tributed by a machine it runs .so freely that it is apt to 
be spread too thickly. This objection may soon be sur- 
mounted by the manufacturers, and, in any case, it could 
probably be overcome by mixing the manure with an equal 
bulk of fine dry soil. To compete successfully with the 
Chilian nitrate it will probably have to be offered at a more 
tempting price. At present the difference is about 2 
per ton in favour of the nitrate of lime, but this corres- 
ponds to only a small difference in the price per unit of 
nitrogen. The Norwegian Hydro-electric Company of 
Christiania produced 1,059 tons of " lime nitrogen," as 
it is sometimes called, in the first three months of last year 
(1908), and this was more than the total output for the 



NITEOGENOUS MANUBES 271 

whole of the previous year. Numerous other companies 
have since been formed to exploit the manufacture. 

NITROLIM (CALCIUM-CYANAMIDE). 

Calcium carbide, which is now extensively used for 
the preparation of acetylene gas, is manufactured on a 
large scale by heating lime and carbon together in an 
electric resistance furnace. If nitrogen be introduced at 
a very high temperature, it also enters into combination, 
and a compound called calcium cyanamide is formed. 

According to the formula, CaCN 2 , ascribed to it, the 
pure substance contains 35 per cent, of nitrogen, which 
is equal to 42*5 per cent, of ammonia. Until a few 
years ago all substances of this class were supposed to 
be poisonous to vegetation, and quite unsuitable for 
mammal purposes. It has been found, however, that 
calcium-cyanamide in aqueous solution is slowly hydro- 
lysed ; the calcium is separated as calcium hydrate, and 
the nitrogen is converted into urea, ammonia and other 
similar products, which are quite innoxious. In the soil 
this change takes place more rapidly, and appears to be 
facilitated by the presence of organic matter. The 
changes also proceed further ; the calcium hydrate is 
converted into the carbonate; amide bodies, e.g., urea, 
are completely hydrolysed to ammonium carbonate, which 
undergoes nitrification, and the nitrogen is thus ulti- 
mately converted into nitrates. The presence of calcium 
in the form in which it exists in the compound, thus 
imparts a certain basic character to it. This has a 
beneficial effect on the properties of the soil generally, 
and is particularly useful in promoting the nitrification of 
the ammonium compounds produced by the hydrolytic 
changes. Assuming that calcium carbonate and ammon- 
ium carbonate are the natural end of the first stage, the 



272 SOILS AND MANURES 

change may be represented by the following equa- 
tion : 

CaCN 2 + 4H 2 + C0 2 = CaC0 3 + (NH 4 ) 2 C0 8 

Calcium Water. Carbon Calcium Ammonium 
cyanamide. dioxide, carbonate. carbonate. 

Calcium-cyanamide is the principal compound in the 
new nitrogenous manure sold under the popular name of 




FIG. 33. View of the Tysselfaldene Power House. 1 

" Nitrolim." This manure has now been m use experiment- 
ally for about five years, and its practical utility is no 
longer open to question. Eleven different companies, with 
a total working capacity of about 150,000 tons a year, 
have been formed to exploit it in various parts of the world. 
The North Western Cyanamide Company, by arrangement 

1 Copy supplied by The North Western Cyanamide Co. 



NITEOGENOUS MANUBES 273 

with the other companies interested, have secured a mono- 
poly for supplying this country. They have erected plant 
for the manufacture of the product at Odda, in Norway, 
where, owing to the precipitous nature of the country, the 
water power employed for working the electrical apparatus 
can be easily and cheaply obtained. 

In appearance, nitrolim is a heavy dark-coloured sub- 
stance somewhat resembling basic slag. To prevent it 
from becoming damp by absorption of moisture from the 
air calcium cyanamide being very deliquescent the 
manure is put up in strong double bags, of which the 
inner one is lined with a special preparation. Under 
these conditions it can be stored for a considerable length 
of time without danger or sensible loss of fertilising 
properties. 

Composition and General Properties. The following 
analysis shows the composition of a sample of nitrolim 
recently examined by Dr. Voelcker : 

Per cent. 

1 Calcium cyanamide ....... 58*91 

Caustic or free lime ....... 23'55 

Magnesia . . . . . . . . . 0*05 

Oxide of iron and alumina ...... 2 P 44 

Siliceous matter ........ 2'19 

Carbon . 12 -86 



100-00 

1 Containing nitrogen 20*62, equal to ammonia 25 '03. 

It will be seen that the manure consists practically of 
a mixture of calcium-cyanamide and lime, together with 
some 15 to 20 per cent, of impurity. Besides the free 
lime, the calcium-cyanamide contains 50 per cent, of 
calcium, which though present in a state of combination, 
is ultimately converted into calcium carbonate in the 
soil, and may therefore be regarded potentially as free 

S.M. T 



274 SOILS AND MANUKES 

lime. One cwt. of this manure is, therefore, equivalent 
to nearly 75 Ibs. of quick lime. 

The proportion of nitrogen is practically the same as 
that in sulphate of ammonia ; the standard product is 
guaranteed to contain '20 per cent, of nitrogen, equal to 
24*5 per cent, of ammonia. 

Nitrolim may be safely mixed with basic slag, bone 
meal, potash salts, and even with superphosphate, if a 
quantity of water be sprinkled over it in the course of 
the operation. But if this does no harm to the calcium- 
cyanamide, the same cannot be said of the superphos- 
phate, and the writer would advise that the two manures 
should not be mixed before application. 

Application. When in good condition, the manure is 
in the form of an extremely fine powder, which, in windy 
weather, is apt to be blown about. It is therefore better 
to apply it by means of a distributor, or, if such a machine 
be not available, it should be mixed with a quantity of 
soil. If basic slag or potash salts are to be spread at the 
same time, they will serve the same purpose, and a certain 
amount of trouble may be saved in that way. 

In order to avoid risk of loss by evaporation of ammonia, 
it is desirable that the manure should be well harrowed 
in, especially on the lighter class of soils. For this 
reason also, if it is to be used as a top dressing, it is 
desirable to mix it with earth before spreading. 

Some observers have recommended that it should be 
applied at the back end of the year, but as the nitrogen 
is present in a very soluble condition, one would not 
expect this to prove an economical method. The majority 
of experiments appear to indicate that the best results are 
obtained with this manure when it is applied about two 
or three weeks before the seed. 

It is suitable for all crops that require nitrogenous 
manures, and may be used at the rate of from one 



NITKOGENOUS MANURES 



27,5 



to two cwts. per acre. As, however, it seems desirable 
to bury the manure in the soil, it is probably better 
adapted for cereals, roots, etc., than for grass lands. 
It has also produced good effects on garden crops. The 
following results obtained in the experimental trials 
afford an indication of the manurial efficiency of 
nitrolim : 

TABLE SHOWING THE EFFECTS OF EQUAL QUANTITIES OF NITROGEN 
IN THE FORM OF NITROLIM AND OTHER NITROGENOUS MANURES 
ON THE GROWTH OF CROPS. 

PER ACRE. 











Sulphate of 







Nitrolim. 


Sulphate of 
Ammonia. 


Nitrate of 
Soda. 


Ammonia and 
Nitrate of 


No 
Nitrogen. 










Soda. 






tons. cwt. 


tons. cwt. 


tons. cwt. 


tons. cwt. 


tons. cwt. 


Mangolds 1 


35 13 


34 5 





34 4 




Turnips 2 . 


25 14^ 








25 18J 




Q A sf 1905 
Swedes* j 1906 


16 2^ 
21 18 


16 
21 6 


14 6 
21 1 






Potatoes 4 


6 18f 





6 18 





6 6 









Nitrolim. 


Sulphate of 
Ammonia. 


Nitrate of 
Soda. 


No 
Manure. 






Lbs. 


Lbs. 


Lbs. 


Lbs. 


Oats 5 . | 


Grain 
Straw 


3,950 
4,670 


3,940 

4,480 


3,790 
4,730 


3,580 
3,800 


Barley 5 . j 


Grain 
Straw 


2,060 
2,492 


2,180 

2,856 


2,160 
2,968 


1,410 
2,184 



1 Cambridge University Farm. 

2 West of Scotland Agricultural College. 

3 Harper- Adams Agricultural College. 

4 Marburg Experimental Station. 

5 Highland and Agricultural Society of Scotland. 



T2 



CHAPTER XI 

POTASH MANURES 

MOST of the ordinary crops remove considerable quanti- 
ties of potash from the land, and any deficiency of this 
constituent seriously impedes their growth. Natural 
deficiency of potash is, however, of comparatively rare 
occurrence, and under ordinary conditions of farming, a 
large proportion of what is taken out of the land in the 
crops, is returned in the farmyard manure. Potash 
manures are not, therefore, so generally necessary, and 
are not so extensively used as other kinds of artificial 
manure. Nevertheless they often have a favourable 
effect, and in some cases are practically indispensable. 
They are all the more necessary since the use of phos- 
phatic and nitrogenous manures has become so common. 
Formerly, when no artificial manures were used, and 
the sale of produce was more restricted, there was com- 
paratively little loss. Nowadays, larger crops are raised, 
more is taken out of the land, and if the margin of 
available potash be naturally small, it may easily be 
overpassed and productiveness thereby reduced. 

Potash. The term potash, as now used in agricultural 
chemistry, always refers to the oxide of potassium (K 2 0). 
In analyses of soils, manures, etc., the total quantities 
of potassium compounds found are always so expressed. 
Such a statement does not necessarily imply that the 
potassium compounds exist in that form in the substance 
analysed. For example, it may be said that a sample of 



POTASH MANURES 277 

potassium chloride contains 63 per cent, of potash. In 
reality it contains none at all, but 63 parts of the oxide 
contain the same amount of potassium as 100 parts of 
the chloride, and are therefore equal to that amount. 
Potassium sulphate and other oxysalts may be said to 
contain the oxide, K a O, in combination -with the acid oxides, 
S0 3 , etc. 

SOURCES OF POTASSIUM COMPOUNDS. 

The principal sources of potassium compounds are 
potassic minerals, plant ashes, and the natural deposits of 
potash salts. 

Potash-bearing Minerals. The most important potash- 
bearing minerals are the potash felspar, orthoclase, and 
the white or potash mica. The former contains from 
12 to 17 per cent, of potash, and the latter from 8 to 13 
per cent. They enter largely into the composition of 
some of the crystalline rocks, and are the ultimate source 
of the potash in soils. They are both silicates (p. 21), 
decompose very slowly, and cannot be used directly as 
manures though both have been tried. The use of mi- 
caceous sand has been previously referred to. Felspar 
has been tried in a finely-ground condition, but not with 
much success. More recently, attempts have been made 
to utilise it by fusing it with sodium carbonate and 
phosphorite (p. 235). The treatment is said to render 
the potash more readily available, but the products are 
regarded more as phosphatic than as potash manures. 

Wood Ashes. This was formerly the principal, and _is 
still a considerable source of potassium compounds. The 
potassic minerals of the rocks from which the soil is 
derived are gradually decomposed ; the potash becomes 
available, is taken up by plants, and forms a large 
proportion of the ash which remains when the organic 



278 SOILS AND MANUEES 

matter is burned off. The twigs and young boughs are 
the richest, and being useless as timber, they are often 
burned for recovery of the potash. The operation was 
formerly carried out in an iron pot hence the name 
pot-ashes but is now usually done in pits^ both in 
Canada and the United States. The ash contains 
from 5 to 10 per cent, of potassium carbonate which 
can be largely separated from the other ingredients by 
lixiviation with water. The product obtained on re- 
crystallisation is called crude potash, and contains from 
50 to 60 per cent, of potassium carbonate. By a further 
process of lixiviation, a purer substance containing up- 
wards of 90 per cent, can be produced. This is called 
pearl ash or American ashes. 

For manurial purposes it would be no advantage to 
separate the potash from the other ingredients, but quite 
the reverse. The ash includes usually from 2 to 4 per 
cent, of phosphoric acid, and 30 to 40 per cent, of car- 
bonate of lime, and both of these constituents, as well as 
some of the potash, would be lost. 

The ashes of other plants and all kinds of vegetable 
refuse may also be used as a source of potassium com- 
pounds, and as potash manure. The ash of seaweed 
has been used to a considerable extent in Scotland, under 
the name of ''kelp." For manurial purposes, however, 
it is an extremely wasteful process to burn the seaweed 
or any other vegetable matter, as the nitrogen is thereby 
dissipated and lost. If the material can be ploughed 
into the soil direct, or can be reduced to suitable con- 
dition by composting, it is much more economically 
employed in that way. Apart from the plant food it 
contains, the organic matter generally has a beneficial 
effect on the physical properties of the soil. The influ- 
ence of potassium carbonate, on the other hand, is highly 
deleterious. It has a strongly alkaline reaction, causes 



POTASH MANUEES 279 

defloculation of clay, and its effects on seeds, if brought 
into contact with them, would be very injurious. The 
effects of plant ashes are, of course, much milder because 
they are not so concentrated, and also because the pres- 
ence of the lime would tend to correct the defloculating 
effects of the alkali. Noxious weeds collected in the 
process of cleaning the land should, of course, always be 
burned, as that is the only way to make sure of destroying 
them. The quantity is usually so small that the loss of 
nitrogen is inconsiderable, and so also are the effects- 
beneficial or otherwise of the ashes which are spread on 
the land. 

POTASH SALTS. 

Occurrence. The extensive saline deposits which were 
discovered in 1839, near Stassfurth, in Germany, were at 
first, worked only for common salt. The occurrence of 
layers, consisting of potassium chloride and other com- 
pounds, was not suspected until some twenty years later. 
It was known that potassium, magnesium and calcium 
compounds were present in the salt in considerable quan- 
tities, but as these substances are commonly found in 
deposits of rock salt they were regarded merely as ordin- 
ary impurity. The importance of the discovery was at once 
apparent. The potash salts have a much higher value than 
common salt. The mines are now worked chiefly for the 
former, and it is the latter that is regarded as impurity. 
All other sources of potassium compounds, both for 
agricultural and other purposes, have since become rela- 
tively insignificant. 

The deposits extend over an area of many square 
miles, and are estimated to be, in places, upwards of 
5,000 feet thick. The supply may be therefore regarded 
as practically inexhaustible, and since the geological 



280 SOILS AND MANUEES 

conditions of their formation have heen determined, it 
is considered not unlikely that other similar deposits 
may be discovered elsewhere. 

Formation of the Deposits. The origin of the deposits 
is attributed to the isolation of bodies of sea-water as a 
result of changes in the elevation of the land. Thus cut 
off from the main ocean, the water evaporated and the 
salts crystallised put. Under these conditions the 
salts would crystallise in the order of their solu- 
bilities, gypsum first being the least soluble - 
then common salt, magnesium sulphate, and lastly, 
the chlorides of potassium and magnesium. With 
various intermediate layers they are found practically in 
that order. Like other rocks, however, they appear to 
have been subjected to various geological changes since 
they were first deposited, and different layers and sec- 
tions have been at various times exposed to the action 
of the weather. Probably some of the salts were dissolved, 
and so brought into contact with those in other layers. 
Eeactions appear to have taken place between them, and 
secondary deposits, e.g., Schonite, kainite, etc., have 
been formed. The whole is now covered up with layers 
of clay, sand and limestone, with rock salt and gypsum 
sandwiched between them, which have for a long time 
protected it from further denudation. Below this lies 
the main deposit in which five principal divisions are 
generally recognised. Taken from above downwards, 
these are as follows : 

1. The Carnallite region, containing potash and mag- 
nesium salts. 

2. The Kieserite region, in which the principal salt 
is magnesium sulphate. 

3. The Polyhalite region, containing a mixed salt, 
consisting chiefly of the sulphates of potassium, mag- 
nesium and calcium. 



POTASH MANURES 281 

4. The Kock salt region, consisting chiefly of sodium 
chloride. 

5. The Calcium region, consisting chiefly of gypsum 
and anhydrite, both sulphates of calcium. 

The following are the more important forms of potash 
salts produced at the mines : 

Sylvine. This consists of practically pure potassium 
chloride or muriate of potash (KC1). It appears to be a 
secondary product derived from the carnallite, and is 
sometimes prepared from it artificially. Commercial 
varieties contain from 70 to 95 per cent, of pure potas- 
sium chloride. 

Sylvinite. This may be regarded as an impure form 
of sylvine, but is of more variable composition. It con- 
tains from 20 to 25 per cent, of potash, chiefly as 
chloride, about 40 per cent, of common salt, and smaller 
quantities of other impurities. 

Carnallite. Carnallite is a well-known double chloride 
of potassium and magnesium. In the pure state its 
composition corresponds to the formula (MgCl 2 , KC1, 6H 2 0). 
The potassium chloride can be separated from the mag- 
nesium salt by a process of crystallization, and is sold 
as muriate of potash. 

Schonite. This salt may be regarded as the sulphate 
corresponding to carnallite. It is represented by the 
formula (MgS0 4 , K 2 S0 4 , 6H 2 0). It is >ot a large pro- 
duct in the mines, but a substance of similar composition 
can be prepared from kainite by dissolving out the 
sodium and magnesium chlorides. It contains about 
50 per cent, of sulphate of potash and a considerable 
quantity of magnesium sulphate, but little or no 
chloride. It is sometimes calcined, i.e., heated, to 
drive off the water, which in the natural condition, 
usually forms about 10 per cent, of the whole. This 
niakes it more concentrated anid saves cost in transport, 



282 SOILS AND MANURES 

By a further process of purification, a large proportion of 
the magnesium sulphate can be eliminated and practi- 
cally pure sulphate of potash produced. 

Kainite. The composition of the pure mineral is repre- 
sented by the formula (K 2 S0 4 , MgS0 4 , MgCL, 6H 2 0) but 
as obtained from the mines, it is always associated with 
a considerable quantity of common salt and other im- 
purities. The proportion of potash is usually about 
10 or 12 per cent., which is equal to 18 or 20 per 
cent, of sulphate of potash. By submitting it to a 
process of lixiviation, a large proportion of the chlorides 
of sodium and magnesium can be eliminated. This in- 
creases the proportion of potash in the residual salt and 
otherwise improves its quality as manure, but it is not 
necessary for ordinary agricultural purposes. 

Polyhalite. This is a substance of similar composi- 
tion, but contains no chloride. Tbe pure substance may be 
represented by the formula (K 2 S0 4 , MgS0 4 , 2CaS0 15 H 2 0). 
The commercial product contains about 25 per cent, 
of sulphate of potash. 

Kieserite. The pure mineral consists simply of mag- 
nesium sulphate and contains no potash at all. The 
commercial product, however, is usually mixed with a 
quantity of carnallite, and contains about 6 per cent, of 
potash in the form of chloride. 

The tables on p. 283 show the average composition of 
the more important potash salts produced at the mines. 

The total output of potash salts from the mines amounts 
to nearly a million and a half tons annually, and of this 
quantity, kainite and sylvinite together form about two- 
thirds. The imports into the United Kingdom are esti- 
mated at upwards of 40,000 tons annually, including 
some 25,000 tons of kainite and sylvinite. 

Manurlal Value. Kainite is distinctly the most popular 
source of potash for agricultural use, It is not, per- 



POTASH MANUKES 



283 



haps, the best adapted for the purpose, but it is one of the 
most plentiful, and therefore the cheapest of the potash 

AVERAGE COMPOSITION OF GERMAN POTASH SALTS. 






Muriate of 

Potash. 


Sylvinite. 


Carnallite. 


Kieserite. 




Per cent. 


Per cent. 


Per cent. 


Per cent. 


*Potash 


51-7 


24-5 


10-7 


6-4 


*Soda .... 


1-7 


23-2 


12-2 


13-7 


Magnesia . 


2-6 


2-9 


13-6 


12-3 


Lime .... 


1-8 


0-8 


0-8 


0-6 


Sulphuric acid . 


2-4 


7-7 


9-7 


13-7 


Chlorine 


43-4 


44-9 


38-7 


32-6 


Water 


3-8 


5-9 


22-5 


26-7 


*Equal to : 










Potassium chloride . | 81*9 


38-8 


16-9 


10-6 


Sodium chloride 


3-2 


43-7 


23-0 


25-8 






Sulphate of 
Potash. 


Schonite. 


Polyhalite. 


Kainite. 


*Potash 


Per cent. 
50-4 


Per cent. 
28-2 


Per cent. 

13-2 


Per cent. 
11-4 


*Soda .... 


0-5 


0-4 


0-5 


18-0 


Magnesia . 


1-4 


11-0 


6-6 


10-4 


Lime .... 





0-8 


20-2 


0-8 


Sulphuric acid . 


43-8 


46-1 


54-0 


20-2 


Chlorine 


2-3 


1-2 


0-6 


31-1 


Water 


1-8 


12-5 


4-8 


14-3 


*Equal to : 










Potassium sulphate . 


93-2 


521 


24-4 


21:1 


Sodium chloride 


0-9 


0-7 


0-9 


33-8 



salts. It is obvious that the purer and more concen- 
trated products prepared from it cannot be sold at the 
same rate, per unit of potash, as the crude mineral. 
The manurial value of kainite and of the other potash 



284 SOILS AND MANURES 

salts depends solely upon the amount of potash they 
contain. It has been suggested that the presence of the 
magnesium compounds may add to their efficiency, but 
there is no experimental evidence to support this view, 
and it is theoretically possible only in the very rare 
cases in which the soil is deficient in magnesium salts. 

Influence of Chlorides. The presence of a large amount 
of common salt in kainite makes it specially suitable 
as a manure for mangolds, but for some other crops it 
is considered highly objectionable. Chlorides certainly 
have a deleterious effect on delicate plants, e.g., many 
flowers and fruits, and for horticultural work it is better 
to avoid any risk of this kind by employing sulphate of 
potash or the calcined sulphate of potash and magnesia. 
As operations are generally on a much smaller scale, 
the difference in price is of less importance. On most 
of the common agricultural crops, the effects of chlorides 
are deleterious rather than beneficial, but the quantities 
present in an ordinary dressing of kainite have very 
little effect of any kind. It may be noted in this con- 
nection, however, that for the same quantity of potash, 
a dressing of kainite contains a much larger quantity of 
chlorine than one of muriate of potash. The former, it 
is true, contains a smaller proportion of chlorine, but 
as it contains only about a fourth or a fifth of the potash, 
much larger quantities of the manure must be employed. 

Comparison of Sulphate and Muriate. Muriate of 
potash is sometimes compared unfavourably with sul- 
phate of potash, but the difference between them has 
probably been exaggerated. The former is more soluble 
and often contains a larger proportion of potash. The 
presence of chlorine is perhaps a disadvantage for cer- 
tain purposes, but it is obvious that this cannot apply 
to circumstances in which kainite can be used with 
safety. If the sulphate can be obtained at the same 



POTASH MANUKES 285 

price per unit, preference should perhaps be given to 
it, but for most ordinary purposes there is probably very 
little to choose between them. 

Soils Suitable -for Potash Manures. Clays and loamy 
soils are usually fairly well supplied with potash, but 
sandy, calcareous, and humus soils, are often deficient 
in that ingredient. It would, however, be a mistake 
to proceed upon the assumption that such is neces- 
sarily and invariably the case. The question can only 
be determined by experiment, and every farmer who 
has not already done so, would be well advised to 
make such a trial, as very surprising results are often 
obtained. The presence of a certain amount of lime is 
essential to the proper action of potash manures, and if 
the soil does not contain enough, more should be added 
before they are applied. It is sometimes found that 
potash manures give a much better result on one side of 
a hedge than on the other. In some cases the difference 
has been traced to the presence of a larger proportion of 
lime in one of the two soils which were in other respects 
very similar. Lime reacts with the salts, liberating the 
potash and soda, and forming calcium chloride and sul- 
phate (p. 116). 

Crops which require Potash Manures. Potash manures 
are generally suitable for all kinds of leguminous crops, 
roots especially mangolds potatoes, and various garden 
crops, e.g., carrots, onions, fruits, etc. For cereals and 
grass they are not, as a rule, so necessary. The effect 
of potash manures on grass lands is often more con- 
spicuous in the improved quality of the herbage than in 
the increased weight of crop. They are particularly 
favourable to tho development of clovers which are often 
lacking in the light land pastures. 

Application. Kainite is commonly used at the rate 
of from 4 to 8 cwts. per acre, and other salts in like 



286 SOILS AND MANUEES 

proportion. From 1 to 2 cwts. of sulphate or muriate 
would contain about the same quantity of potash. These 
manures may be applied either at the back end of the 
year or in the spring. Though soluble in water they 
are not very liable to loss by drainage. If kept until 
the spring they should be put on as early as possible 
say about the same time as basic slag in order to allow 
of their becoming thoroughly mixed with the soil. They 
may be mixed with any of the nitrogenous manures or with 
superphosphate. They may also be mixed with basic 
slag just before application, but the mixture should not 
be allowed to stand. It is better not to mix them. 



CHAPTER XII 

COMPOUND AND MISCELLANEOUS MANURES 

COMPOUND MANURES 

IN addition to the simple substances described in the 
previous chapters, a variety of fertilisers of more complex 
origin and character can also be obtained. Some of them 
are prepared from scrap and refuse materials that are 
unsuitable for direct use, the plant foods being rendered 
available by treatment with acids and by other pro- 
cesses. Others are made by merely mixing simple 
manures together in various proportions. Most com- 
monly, perhaps, they consist mainly of some simple 
manure mixed with larger or smaller quantities of various 
kinds of scrap. 

Most of them contain phosphates and nitrogen, and a 
few also contain potash. The last are general manures 
and are more or less suitable for all crops alike. In 
some cases the ingredients are blended together in pro- 
portions supposed to be generally most suitable .for par- 
ticular crops. They are called turnip manures, potato 
manures, grass and grain manures, vine manure, and 
so on. Others are described as compound fertilisers, 
artificial guanos, etc., or by some personal or fancy 
name. 

Some of the products are absolutely worthless or 
barely worth the cost of carriage, but these are not now 
so common as they formerly were. Many compound 
manures contain adequate quantities of available plant 



288 



SOILS "AND MANURES 



foods, judiciously mixed together, and are well adapted 
for their ostensible purposes. The following are the adver- 
tised guarantees of some of the mixed manures now on the 
market : 



Manure for 


Soluble 
Phosphate. 


Insoluble 
Phosphate. 


Nitrogen. 


Potash. 




Per cent. 


Per cent. 


Per cent. 


Per cent. 


Turnips 


25 


5 


1 







20 


3 


liy 


3 


Mangolds 


16 


4 


5 


10 




20 


3 


3 


2 


Grain . 


15 


2 


5 


4 




18 


3 


3 





Potatoes 


20 


3 


3 


i* 


" 


18 





3^ 


8 




15 


10 


5" 


12 


Peas and beans 


20 





0-8 


10 




12 


10 


1 


8 


Fruit trees and bushes 


15 


10 


5 


5 


Flowers 


16 


12 


7 


5 



The prices charged by reputable firms are generally 
fixed according to quality, i.e., to the amounts of fer- 
tilising ingredients. Sometimes, however, they are 
ridiculously out of proportion to the cost or value of 
the goods. In any case, it is obvious that compound 
manures cannot be sold at the same rate as the simple 
untreated or unmixed materials from which they are 
prepared. 

The use of compound fertilisers cannot be recom- 
mended from the point of view of economy. The fer- 
tilising ingredients can generally be obtained at a cheaper 
rate in the form of simple unmixed manures, and pro- 
duce quite as good results when applied to the soil 
separately. Some of the constituents of a compound 
manure containing phosphates, nitrogen and potash, are 
bound to be wasted if it is applied indiscriminately to all 



COMPOUND AND MISCELLANEOUS MANUEES 289 

crops alike. Even in the case of those intended for par- 
ticular crops, it is evident that the nature and require- 
ments of the soil cannot be taken into account in pre- 
paring the mixture. Compound manures are, however, 
very convenient for those who, like .gardeners, use only, 
small quantities, and for any who do not understand the 
nature of the different kinds of artificial manures. If 
they are to be employed it will generally be found cheaper 
to buy them from a manufacturer than to attempt to 
prepare them at home. 

MISCELLANEOUS MANURES. 

Common salt, gypsum, copperas and some other sub- 
stances of minor importance which are occasionally 
applied to the soil are commonly classed together as 
miscellaneous manures. They contain no nitrogen, phos- 
phates or potash ; some of them are destitute of any 
essential element of plant food and their action, if any, 
must be attributed to other causes. 

Common Salt. The presence of common salt in kainite 
is considered objectionable because its effects on most 
plants are deleterious rather than beneficial. Some 
crops, it is true, can withstand the application of con- 
siderable quantities, and others, e.g., mangolds, aspara- 
gus, etc., actually appear to thrive on it. Common salt 
consists of sodium chloride, and the deleterious effects 
appear to be due to the chlorine. Other chlorides have 
an equally bad effect ; that of calcium chloride is worse, 
probably because that compound is more soluble. Other 
sodium compounds, e.g., sodium sulphate, seem to 
be harmless. It does not appear that their presence 
in the soil is of any advantage to the majority of crops, 
but it is possible that the beneficial effects of common 
salt on mangolds may be due to the sodium. On the 

S.M. U 



290 SOILS AND MANURES 

other hand, it may be due to some indirect action of the 
salt as a whole. 

Common salt possesses a certain power of flocculating 
clay, but it is not so efficacious as lime. It is slightly 
deliquescent, and the small amount of moisture it attracts 
may possibly be of some benefit to the plants. It has 
been supposed that it renders plant foods in the soil more 
readily available, but this is not supported by any 
satisfactory experimental evidence. By reaction with 
lime it tends to decalcify the soil, and it is said to favour 
"pan " formation. 

The use of common salt has also been recommended 
on the ground that it has an antiseptic action and pre- 
serves the manure (farmyard manure). If it does retard 
the action of manure, it should certainly not be used 
unless the soil is too rich, i.e., contains too much 
nitrogen. It retards the growth of the plants ; thus straw 
is rendered less luxuriant and less liable to be laid, and 
the intense (rank) green colour produced in grass and 
root crops by an overdose of nitrate of soda may be 
corrected. If the soil has become overstocked or over- 
dosed with nitrogenous matter, such a condition can 
sometimes be corrected by using additional quantities 
of phosphates and potash manure ; this enables the 
plant to utilise the excess of nitrogen, and increased pro- 
duction results instead of rankness. Common salt, ap- 
parently, either prevents the plants from absorbing the 
excess of nitrogen or of using it advantageously. The 
principal benefits can therefore be obtained by using less 
manure. 

Salt is sometimes spread on pastures to induce the 
stock, which are very fond of the taste of it, to eat down 
the herbage more closely. Any improvement produced 
in this way cannot properly be regarded as a manurial 
effect. 



COMPOUND AND MISCELLANEOUS MANUEES 291 

Gypsum. This substance may be used to supply 
calcium salts to soils that are deficient in that constituent, 
if any such exist, and it is supposed to be good for legu- 
minous crops. In no sense is it a substitute for lime, 
and the author has never seen any beneficial effect 
produced by it. 

Copperas. It is very difficult to believe that copperas, 
the common sulphate of iron, has any direct manurial 
value. Nevertheless, some remarkable effects have been 
produced, apparently, by the application of this substance 
to potatoes. It has also been recommended for cabbages, 
cereals and grass. As a reducing agent it might be ex- 
pected to prove harmful, and only small quantities 
about -J cwt. per acre must be used. On open soils 
very little danger is to be apprehended from this cause 
as it is rapidly oxidised. The fertilising properties 
claimed for ferrous sulphate may, perhaps, be due to 
catalytic action of the ferric oxide, so formed, upon the 
humus of the soil. 

Magnesium Salts. Magnesium sulphate has been 
occasionally used, but the cases in which this element is 
deficient in the soil are so rare that the subject need not 
be further discussed. Kainite and most of the potash 
salts contain quantities of magnesium compounds. 



u2 



CHAPTER XIII 

GENERAL MANURES ANIMAL AND VEGETABLE 

General Manures. Consideration of the general 
manures has been postponed to the second place simply 
on account of their greater complexity. The arrange- 
ment has no reference to their relative importance as 
compared with the special manures. * The latter, as a 
rule, contain only one fertilising constituent, and their 
virtue depends entirely upon their chemical composition. 
The former are complete manures, i.e., they contain 
phosphates, nitrogen and potash, and the mechanical 
effects produced by some of them are nearly as impor- 
tant as those due to the fertilising ingredients. They are 
all of organic origin and consist of waste animal or vege- 
table matter, or a mixture of the two. Those which 
consist entirely of animal matter are generally the most 
concentrated and most active. Those which consist of 
vegetable matter are more bulky and decompose more 
slowly, but produce greater mechanical effects. The 
mixed products, to which farmyard manure belongs, are 
less concentrated than the jfirst, but are often nearly as 
active, and their mechanical effects are often greater 
than those of the purely vegetable manures. 

ANIMAL MANURES GUANOS. 

Introduction of Guanos. The discovery of guano was 
the immediate cause of what has amounted to a great revo- 
lution in modern agriculture. Its fertilising properties 



GENERAL MANURES ANIMAL AND VEGETABLE 293 

appear to have been long known to the natives of the dis- 
tricts in which the deposits were found, but it was not 
introduced into this country till about the middle of last 
century. The effects produced by it must have appeared 
very striking to the farmers of that time, who, it should be 
remembered, were entirely unacquainted with the highly 
concentrated special manures now in common use. 
Wherever it was tried, it produced results which were 
then regarded as simply marvellous. Eeports of its 
efficiency spread rapidly, and it soon acquired a deservedly 
great reputation throughout the country. Some of the 
accounts which were circulated, however, appear to have 
been much exaggerated. Farmers bought it eagerly, and 
as the supply was limited, highly inflated prices were 
realised. In order to cope with the extraordinary demand, 
the shipments ;were greatly increased, and the then known 
deposits were rapidly exhausted. 

Artificial Guanos. These circumstances naturally gave 
rise to the practice of adulterating the genuine article 
with worthless materials, and to the production of imita- 
tions which often closely resembled it in composition. 
The latter were at first sold as genuine, and afterwards 
as " artificial guano." Some of those which contained 
equal quantities of plant food were found to be not greatly 
inferior in fertilising qualities. They can be sold at much 
lower prices than genuine guano commands, and to repre- 
sent the'm as such, is fraud. The name artificial guano is 
still frequently applied to compound manures, town refuse, 
and even to worthless materials. Pish and meat meals 
are often described as guanos, but the nature of these 
products is now well known and there is little danger of 
anyone being deceived. The writer is accustomed to 
regard with suspicion any fertiliser, other than the 
genuine article, described as "guano." ,The name has 
lost much of its old-time charm, and is not now commonly 



294 SOILS AND MANUKES 

given to manures which can be sold on their merits, i.e., 
on the strength of the proportions of potash, phosphates 
and nitrogen which they contain. That, of course, is 
the ultimate test of all concentrated manures and must 
be applied to genuine guanos as well as to artificial pro- 
ducts of ali kinds. 

Origin and Occurrence. Guanos are found in large 
natural deposits in islands and on the coasts of the main- 
land in various parts Of the world, but chiefly in tropical 
regions. They have been formed mainly of the excre- 
ment of sea-birds which congregate together in colonies, 
especially at certain seasons of the year. Feathers and 
the whole bodies of birds, more or less decayed, are 
usually present in considerable quantities, and the re- 
mains of seals and other large marine animals are also oc 
casionally found. The bulk of the deposits is, however, 
made up of excrement. The word guano is simply the 
Spanish for dung. 

Preservation of the Deposits. Some of the deposits are 
upwards of 200 feet thick and are covered over with 
sand, etc., to a considerable depth. Owing to the com- 
pression thus produced and the dryness of the climates 
in which they are found, comparatively little fermenta- 
tion has taken place. Otherwise .they could not have 
been preserved for they are by no means of recent 
formation and the material of which they consist is 
subject, under ordinary conditions, to very rapid decom- 
position. Fermentation has not, however, been entirely 
arrested, and the materials have undergone slow but con- 
tinuous change since they were first deposited. Uric 
acid, which is the principal nitrogenous compound in 
the excrement of birds, has been largely converted into 
ammonium carbonate and intermediate products, and in 
most cases, a certain amount of nitrification has also 
taken place. A part of the nitrogen and other constituents 



GENEEAL MANTJEES ANIMAL AND VEGETABLE 295 

is therefore immediately available, and the remainder 
readily becomes so. Such conditions are associated with 
the highest degree of fertilising power of any given 
quantity of plant food, and they do not generally, 
obtain in artificial guanos. 

Nitrogenous and Phosphatic Guanos. In moist climates 
fermentation takes place much more rapidly, and a large 
proportion of the nitrogen and other soluble constituents 
is lost. In some cases they have been completely re- 
moved and only the insoluble tricalcic phosphates remain. 
These are the phosphatic guanos previously referred to. 
They are chiefly employed for the manufacture of super- 
phosphates. The latter are sometimes called dissolved 
phosphatic guanos, but they have no greater manurial 
value than the superphosphates prepared from mineral 
apatites and phosphorites. Guanos which contain con- 
siderable, or even appreciable quantities of nitrogen, are 
by contrast, called nitrogenous guanos, but it is to these 
and these only that the unqualified word guano usually 
and properly refers. 

Sources and Quantities. Nitrogenous guanos were first 
discovered in Peru, and the largest quantities are still 
obtained from that country. Deposits were subsequently 
found in various parts of South and North America, West 
Indies, Australia, Pacific Islands, etc., but most of them 
were of inferior quality and some were purely phosphatic. 
Peruvian guanos were mostly of very high grade, and the 
term has sometimes been regarded as synonymous with 
nitrogenous guano. 

The quantity of guano consumed in this country has 
varied considerably from time to time as new deposits 
were discovered and became exhausted. In the year 1870 
it amounted to nearly 250,000 tons, and in 1887 it was 
only a little over 5,000 tons. 

The following table shows the quantity of guano im- 



296 



SOILS AND MANURES 



ported into the United Kingdom from various countries 
in 1907 : 



Country. 

Peru .... 
Venezuela . 
Uruguay 

Seychelles Islands 
Iceland and Greenland 
Other countries . 

Total 



Tons. 

26,107 

1,400 

689 

626 

486 

1,970 

31,278 



Composition. The composition of guanos is extremely 
variable according to the conditions to which they have 
been exposed. In the older deposits a certain loss of 
organic matter by decomposition probably took place, and 
the proportion of phosphates and other ash constituents 
was correspondingly increased, without much loss of 
nitrogen. In most cases the phosphates have increased 
only in proportion to the amount of nitrogenous matter 
removed, and it is generally found that those which are 
richest in phosphates are poorest in nitrogen. 

The average proportions found in some of the well- 
known deposits were as follows : 






Nitrogen. 


Phosphates. 




Per cent. 


Per cent. 


Angamos ..... 
Ballesfcas ..... 


20 
11 


10 
28 


Ichaboe 


9 


26 


Patagonian .... 


4 


40 



All these are entitled to be classed as nitrogenous guanos, 
and if the phosphatic samples be taken into account it 
may be said, in general, that the proportion of nitrogen 
varies from up to 20 per cent., phosphates from 



GENERAL MANURES ANIMAL AND VEGETABLE 297 

10 to 90 per cent., and pc/tash to 5 per cent. The 
richest deposits of nitrogenous guano are all long since 
exhausted, but fairly good samples can still be obtained 
containing from 6 to 12 per cent, of nitrogen, 10 to 20 
per cent, of phosphates, and 1 to 4 per cent, of potash. 

Manurial Value. - - Guanos are highly concentrated 
manures and have little or no effect on the mechanical 
properties of the soil. The manurial value depends 
entirely upon the chemical composition and is generally 
estimated simply by the proportions of plant foods they 
contain. In view of the peculiar and varied states in 
which the plant foods exist in guanos, it is not improbable 
that they have a somewhat higher fertilising value than 
those of compound manures. .Guanos should therefore 
be purchased subject to analysis, and under a guarantee 
that they are pure and genuine. 

Guanos are general manures, but the proportions of 
the different constituents are often not very suitable for 
general agricultural purposes. In order to remedy this, 
quantities of potash, phosphates or nitrogenous matter 
are sometimes added to thenij and the mixtures are called 
" rectified " or " equalised " guanos. The only objection 
to this, from the farmer's point of view, is that no dis- 
tinction is made between the original and the added con- 
stituents, and the latter are often charged at guano 
prices. It is therefore better for the farmer, knowing 
the composition of the guano, to buy the constituents 
in which it is deficient and apply them, to the soil 
separately. As the major part of the phosphates and 
nitrogenous matter are present in an insoluble form, 
guanos are occasionally treated with acid and sold 
as "dissolved guano." This treatment no doubt renders 
the plant food more readily available, and it is claimed, 
greatly improves their manurial value. It completely 
alters the peculiar condition in which the fertilising in- 



298 SOILS AND MANUEES 

gredients exist, and so destroys the only possible grounds 
upon which guanos may be considered superior to the 
ordinary mixed or dissolved manure containing an equal 
quantity of plant food. If dissolved or compound manures 
are to be used, they may as well be made from the 
cheapest materials. 

Application. It is impossible to say for what crops 
guianos are most suitable or what quantities should be 
used, unless their composition is known. The higher 
grade samples, i.e., those which contain a large propor- 
tion of nitrogen, should be applied chiefly to the cereal 
and grass crops ; the more phosphatic varieties are better 
adapted for roots and potatoes. The application of 
5 or 6 cwts. per acre of strong nitrogenous guano to 
turnips can scarcely be justified either on theoretical or 
economic grounds, though such quantities may not be 
too large when they contain relatively small quantities of 
nitrogen. From 2 to 4 cwts. per acre may be advan- 
tageously employed for grass and cereals. Guano should 
be applied in the spring, some time before the seed, and 
should be well harrowed in, in order to avoid risk of loss 
by evaporation of ammonia. When used as a top dressing 
it is advisable to mix a quantity of fine soil with it. Salt 
has sometimes been used for this purpose, but in the 
writer's opinion, such a practice is not to be recommended. 

The quality of guano cannot be judged from its ap- 
pearance. The colour and smel) are fairly character- 
istic, but they can .be easily imitated. The only real 
safeguard is chemical analysis, and in purchasing this 
variable and usually expensive manure ifc should never 
be neglected. 

Sewage. Ordinary sewage consists mainly of human 
excreta mixed with a relatively very large quantity of 
water. It is produced at the rate of about 25 or 30 gallons 
per head per day, and contains from 6 to 8 grains per 



GENERAL MANUEES ANIMAL AND VEGETABLE 299 

gallon of nitrogen, from 2 to 3 of phosphoric acid, and 
from 3 to 4 of potash. These quantities amount to 
about 10 Ibs. of nitrogen, 4 Ibs. of phosphoric acid, and 
5 Ibs. of potash annually, dissolved in from 40 to 50 tons 
of water. The total quantity of fertilising ingredients 
allowed to go to waste in this way, it will thus be seen, 
is very large, but the sewage is so dilute that it cannot 
be profitably employed for manurial purposes. If applied 
in the ordinary way the return for the quantity of plant 
food it contains would not pay for the cost of distribution. 
When applied in large quantities by a system of irriga- 
tion, heavy crops of coarse, rank herbage may be pro- 
duced from very poor or almost worthless sandy soils. 
Such a system is, however, of more importance as a 
means of disposing of sewage than of manuring agricul- 
tural lands. In any case it is too large a question to be 
fully discussed here. 

Sludge Manures. The dried sludge resulting from the 
purification of sewage contains a certain amount of 
plant food, and is often sold as manure. Most of the 
processes employed consist essentially in adding various 
substances to the sewage so as to produce a voluminous 
precipitate ; this is allowed to settle slowly in tanks, and 
carries down with it practically all the suspended matter, 
and usually, also some of the dissolved ingredients. In 
some cases the effluent is afterwards filtered and is 
generally rendered quite clear. The principal object is to 
prevent pollution of the rivers, into which it is finally 
discharged. All the processes hitherto employed fail to 
recover much of the soluble nitrogen which is by far the 
largest and most valuable fertilising constituent. The 
manurial value of the sludge which remains in the set- 
tling tanks depends very largely upon the nature and 
efficiency of the precipitant employed. At the Dalmar- 
nock works, hear Glasgow, the precipitate is produced 



300 SOILS AND MANURES 

by the 'addition of alum and lime. About 10,000,000 
gallons of sewage are treated daily, and about 4J tons of 
sludge manure is produced. The latter contains about 
2 per cent, of nitrogen and 3 J per cent, of tricalcic 
phosphate. Both constituents appear to be readily avail- 
able, and the manure fetches a fairly high price. In 
some cases a mixture of alum, blood and clay is used as 
a precipitant. The nitrogenous matter of the blood is 
added to the sludge, and a manure of different composi- 
tion is produced. 

Liquid Manure. Liquid manure consists of the urine 
of animals, collected in stables and cowhouses, and the 
drainings from the yard manure heaps. It is valuable 
chiefly for the nitrogen and potash it contains, the pro- 
portion of phosphates being very small. The composition 
of urine is very variable. It depends, to a large extent, 
upon the quantity of water and the kind and quantity 
of food consumed by the animal. Under normal condi- 
tions the urine of horses and cattle contains from 87 to 
97 per cent, of water, from 1 to 1.5 per cent, of nitrogen, 
and a similar amount of potash. Perhaps the best way 
to utilise it is to pump it over the manure heap. If the 
latter is dry enough to absorb it, the liquid promotes fer- 
mentation, enriches the manure, and has a generally good 
effect. If the heap be too moist to absorb any more, the 
liquid may be made into a kind of compost by mixing it 
with a quantity of humus soil or even ordinary dry soil. 
If it is proposed to utilise the liquid manure in this way 
or to keep it for any length of time, a small quantity 
of sulphuric acid should be added to prevent loss of 
nitrogen by evaporation of ammonia. Liquid manure 
may also be applied to the land direct by means of a 
sprinkler, but should be diluted with two or three times 
its volume of water. In the undiluted state it is apt to 
produce a burning effect. 



GENEKAL MANUEES -ANIMAL AND VEGETABLE 301 

VEGETABLE MANURES. 

Unmixed vegetable matter is not very largely used 
as manure on farms. It can generally be employed 
more economically either as fodder or as litter for 
animals. It contains comparatively small proportions of 
fertilising ingredients ; these only become available as 
the organic matter decays, and decomposition, as a rule, 
takes place somewhat slowly. Vegetable manures are 
generally very bulky, and produce considerable mechani- 
cal effects. 

Green Manures. Crops which are neither cut nor 
eaten off, but simply ploughed into the land in the fresh 
condition are called green manures. They add little 
or nothing in the shape of plant foods, but they increase 
the quantity of humus and are chiefly valuable for the 
effects of the latter upon the physical properties of the 
soil. It is to be noted that though all, or nearly all, 
the fertilising ingredients in green manures are derived 
from the soil, a certain proportion may have come from 
the lower depths, and helps to enrich the surface soil. 
Green manures may also be useful as a means of prevent- 
ing loss of nitrogen ; the crops used for green manuring 
are generally raised as " catch crops " in the autumn, and 
are thus able to take up the nitrates produced during the 
late summer and which would otherwise be liable to be 
washed out of the soil by the winter rains. When taken 
up by the crops, the nitrogen is converted into insoluble 
organic compounds which must undergo nitrification be- 
fore it again becomes available. As the time available 
for the growth of an autumn catch crop is short, quick- 
growing plants, e.g., mustard, rape, etc., are considered 
most suitable for the purpose. If a leguminous crop, 
such as clover, vetches, lupines, etc., be used, a con- 
siderable quantity of nitrogen may be actually added to the 



302 SOILS AND MANURES 

soil by this means. The physical properties of very light 
and very heavy soils may be greatly improved by green 
manuring, but under ordinary circumstances, unless the 
land is conspicuously deficient in humus, it is generally 
more profitable either to cut the crop or to feed it off. 
In the latter case most of the plant foods are returned 
to the land, but as the organic matter is consumed Jby 
the animals, the land is not greatly enriched in humus. 
If the crop is carried off the land, the effect is to im- 
poverish the soil as previously explained. This, of 
course, is the reverse of manuring. 

Seaweed. The nitrogenous matter and other plant 
food that is constantly being poured into the sea in 
sewage and in other ways is not altogether lost. It is 
taken up by marine plants, and considerable quantities 
of it may be recovered in this form and used as manure. 
The manurial value of seaweed is no modern discovery. 
It is, and has long been, used in large quantities on 
farms near the coast, but being very bulky, it cannot be 
profitably carried any distance inland. In the fresh 
state it contains a large proportion of water, and the 
cost of transportation can be greatly reduced by dry- 
ing it, when it is possible to do so, before carting. 

It appears from numerous analyses which have been 
published that the composition varies considerably. The 
general averages are as follows : 

Per cent, 

Water . 70 80 



Organic matter 
Ash 

Nitrogen 
Phosphoric acid 
Potash . 



10 20 
5 10 
0-4 0-8 
0-1 0-2 
1-0 2-0 



Compared with farmyard manure the fresh substance 
generally contains about the same quantity of nitrogen, 
and is rather richer in potash, .but poorer in phosphates. 
The tissues are soft and cellular ; it decomposes rapidly 



GENERAL MANUEES ANIMAL AND VEGETABLE 303 



in the soil and produces considerable mechanical effects. 
The large proportion of chlorides in seaweed has been 
urged as a theoretical objection to its use, but practi- 
cally it has been found 1 to produce quite as good results 
as an equal quantity of farmyard manure. In some 
districts it is much esteemed as manure for roots, es- 
pecially mangolds, but as it is somewhat deficient in phos- 
phoric acid, much better results are obtained when a 
quantity of phosphatic manure is applied in conjunction 
with it. 

Waste Cakes and Feeding Stuffs. Some oil cakes are 
unfit for feeding purposes owing to the presence of 
poisonous ingredients, e.g., castor and mustard seeds, 
rancid oils, etc. Cargoes of grain and other feeding 
stuffs are sometimes damaged by sea water, become 
mouldy, or are otherwise spoilt. All these are fairly 
rich in nitrogenous matter and other plant foods and 
may be used as manure. Cakes are generally the 
richest, but the presence of oil retards their decom- 
position. They contain from 2*5 to 7 per cent, of 
nitrogen, 0*5 to 1*5 per cent, of potash, 1 to 3 per cent, 
of phosphoric acid, and 5 to 10 per cent, of oil. They 
are sometimes de-oiled and ground up very fine. This 
accelerates their action and greatly increases their 
manurial value. 

The quantities of fertilising ingredients in a ton of 
some of the cakes is shown below : 






Nitrogen. 


Potash. 


Phosphoric Acid. 




Lbs. 


Lbs. 


Lbs. 


Linseed cake 


100 


30 


40 


Rape cake .... 


110 


30 


45 


Castor cake 


120 


25 


40 



1 Hendrick. Trans. H. & A, S. (1898). 



304 SOILS AND MANUEES 

Waste cakes, when sold for manure, fetch from 2 10s. 
to 3 per ton. 

Leaf-Mould. Fresh young leaves contain a consider- 
able proportion of fertilising ingredients, but become 
largely exhausted towards the autumn, and when they 
wither and fall are scarcely worth collecting for their 
manurial value. They are not, therefore, much used for 
agricultural purposes, but in gardens where they are 
gathered up for the sake of appearances, they are gener- 
ally made into a kind of compost and used as manure. 
The ash consists largely of silica and the organic matter 
of cellulose which decomposes very slowly. A sample 
of beech leaves was found to contain 1*1 per cent, of 
nitrogen, 0'3 per cent, of potash, and a like amount of 
phosphoric acid, but the proportions of fertilising in- 
gredients found in samples of mixed leaves are generally 
somewhat smaller. Leaf-mould is described by gardeners 
as a mild manure ; it is probably more valuable for 
its physical properties than for its chemical composition. 

A fairly rich manure somewhat resembling leaf -mould 
can be made from ferns, in districts where they are 
plentiful. For this purpose the plants should be cut 
fairly young, as in that condition they contain a larger 
proportion of plant food and decompose more readily. 
The difference in composition between the young and the 
old plants is shown by analyses made by Mr. J. Hughes, 
as follows : 






Young Fern. 


Old Fern. 


Nitrogen ..... 
Phosphoric acid .... 
Potash 


Per cent. 
2-42 
0-60 
1-15 


Per cent. 
0-9 
0-3 

o-i 



For agricultural purposes leaves and ferns are some- 
times used as litter. 



CHAPTER XIV 

FARMYARD MANURE 

Ix many respects farmyard manure is the most im- 
portant of all manures. It is produced in the ordinary 
course of farming, and consists essentially of those parts 
of the crops that are unsuitable for use as foods, mixed 
with the droppings of the animals. It is the medium 
by which the surplus plant foods taken from the soil 
are restored to it. The fertility of the land is thus 
maintained at its normal level, but cannot generally be 
increased beyond that point by this means. The avail- 
able plant foods derived from a given area of land during 
the rotation are, however, concentrated in the manure 
and can be applied in large quantity to those crops 
which most particularly require them. The fertilising 
effects of farmyard manure are well known and are all 
the more conspicuous because it is usually applied to 
land which has become partially exhausted during the 
rotation. The essential function of farmyard manure is 
restoration, and previous to the introduction of the so- 
called artificial fertilisers and feeding stuffs, it could 
not be employed to increase the fertility of one piece of 
land beyond a certain point without reducing that of 
another in a corresponding degree. When these sub- 
stances are used, the farmyard manure has naturally a 
much wider scope. 

Within certain limits the composition of farmyard 
manure is very variable. The causes and extent of this 
variation are amongst the most important points to be 

S.M. x 



306 SOILS AND MANUEES 

considered in this connection. Briefly, the composition 
depends mainly upon the following conditions: 

1. The composition of the animal excreta. 

2. The kind and quantity of the litter. 

3. The nature and degree of fermentation. 

4. The amount of loss in the process of making. 

THE EXCRETA. 

The Dung. The dung, or solid excrement, consists 
mainly of the surplus and indigestible portions of the 
food. It contains also some of the digestive juices and 
effete membranes of the alimentary tract, but under 
ordinary conditions, the proportions of these ingredients 
are too small to sensibly affect the composition of the 
manure, and for practical purposes they may be ne- 
glected. 

The composition of the dung is more or less charac- 
teristic of the kind of animal by which it is produced. 
The most striking and perhaps the most important differ- 
ences are in regard to the proportion of water. The 
dung of horses, it is well known, is much drier and fer- 
ments more rapidly than that of cows. Horse dung 
is, for this reason, more suitable for the preparation of 
hotbeds, and, for manurial purposes, is generally pre- 
ferred to an equal weight of cow dung. Horse and cow 
manures can often be obtained from town stables and 
milk farms respectively, but on an ordinary farm they are 
not usually kept separate, and the character of the heap 
depends partly on the proportions in which they are 
mixed. The quantity of manure produced by pigs and 
other animals on an ordinary farm, is not usually suffi- 
cient to affect the composition of the heap to any great 
extent, but if it formed a large proportion, it certainly 
would do so. The composition of the dung of common 



FAEMYAED MANUEE 



307 



farm animals, according to Stoeckhardt, is shown in 
the following tables: 

AVERAGE COMPOSITION OF THE FRESH DUNG OF FARM ANIMALS. 






Horses. 


Cows. 


Sheep. 


Pigs. 


Per cent. 


Per cent. 


Per cent. 


Per cent. 


Water .... 


76 


84 


58 


80 


Organic matter 


21 


13-6 


36 


17 


Ash .... 


3 


2-4 


6 


3 




100 


100-0 


100 


100 



Nitrogen 




50 


30 


75 


60 


Phosphoric 


acid . 


35 


25 


60 


45 


Alkalis . 


. 


30 


10 


30 


50 



THE SAME CALCULATED TO THE DRY STATE. 



Nitrogen 


2-08 


1-87 


1-78 


3-00 


Phosphoric acid . 


1-45 


1-56 


1-42 


2-25 


Alkalis 


1-25 


062 


0-71 


2-50 



The composition of the solid matter of the dung is 
affected to a considerable extent by the character and 
quantity of the food supplied to the animals. 

The differences in the diets that are customary and ap- 
propriate for the several kinds of animals, probably 
account, in part at least, for the differences observed 
in the composition of the dry substance of their dung. 
Minor variations in the composition of the dung of each 
kind of animal can also be traced to this cause. It is 
well known that when the diet includes a quantity of 
grain, cake and other highly nitrogenous substances, 

x 2 



308 SOILS AND MANUEES 

the manure is necessarily much richer than when the 
animals are fed entirely on coarser and more bulky 
fodders, such as hay, straw and roots. It will presently 
be shown, however, that this is due mainly to the differ- 
ence in the composition of the urine. 

The Urine. The solid constituents of the urine or 
liquid excrement are derived directly or indirectly from 
the digested portion of the food which is absorbed into 
the blood and is used to form the tissues. Owing to 
various physiological changes which take place, only a 
comparatively small proportion of the nitrogen and ash 
ingredients are permanently retained by the animal. 
The amount varies according to circumstances, but by far 
the largest part is ultimately discharged in the urine. 

In the case of horses and store animals, practically 
the whole of the fertilising ingredients of the digested 
portion of the food passes into the urine. Growing and 
fattening animals retain a certain amount wherewith to 
build up new tissues, and a still larger proportion is 
utilised in the production of milk by cows and other 
animals. Whatever is recovered in the shape of animal 
produce, e.g., meat, milk, wool, etc., is lost to the manure. 

The relation between the food consumed and the pro- 
portions of fertilising ingredients voided in the dung 
and urine, and recovered in the shape of animal pro- 
duce, is shown in the tables 1 on p. 309. 

It will be seen that, as a rule, from 60 to 80 per cent, 
of the nitrogenous matter of the food is voided in the 
urine, and from 20 to 30 per cent, in the dung. The 
combined excreta contain from 75 to 95 per cent, of the 
total nitrogen, and from 90 to 98 per cent, of the ash 
ingredients of the food. Phosphoric acid enters into the 
composition of animal tissues and a considerable pro- 

1 Warington ; " Chemistry of the Farm." 



FAKMYABD MANURE 



309 



portion of it is therefore retained. Potash, 1 on the 
other hand, is not apparently withdrawn from solution to 
any great extent, and usually appears in the urine in 
much larger quantities than phosphoric acid. Not only 

NITROGEN IN ANIMAL PRODUCE AND VOIDED, FOR 100 CONSUMED 

AS FOOD. 






Obtained as 
Carcase or 

Milk. 


Voided as 
Solid 
Excrement. 


Voided as 
Liquid 
Excrement. 


In Total 
Excrement. 


Horse at rest 


None 


43-0 


57-0 


100 


Horse at work 


None 


29-4 


70-6 


100 


Fattening oxen . 


3-9 


2:2-6 


73-5 


96-1 


Fattening sheep . 


4-3 


16-7 


79-0 


95-7 


Fattening pigs 


14-7 


22-0 


63-3 


85-3 


Milking cows 


24-5 


18-1 


57-4 


75-5 



ASH CONSTITUENTS IN ANIMAL PRODUCE AND VOIDED, FOR 100 
CONSUMED AS FOOD. 






Obtained as Live 
Weight or Milk. 


Voided in Excrements 
and Perspiration. 


Horse 


None 


100 


Fattening oxen .... 


2-3 


97-7 


Fattening sheep .... 


3-8 


96-2 


Fattening pigs .... 


4-0 


96-0 


Milking cows .... 


10-3 


89-7 



is the total quantity of fertilising ingredients voided in 
the urine much larger than that voided in the dung, 
but even when compared weight for weight, the former 
is often richer than the latter in all except phosphoric 

1 A certain amount of potash is lost in the perspiration of most 
animals, and the sv.int of sheep's wool contains a considerable quantity. 
With this exception, practically the whole of the potash in the 
digestible portion of the food is recovered in the urine. 



310 SOILS AND MANURES 

acid. Moreover, the constituents of the urine are present 
in a soluble state, and are therefore immediately avail- 
able to plants, whereas those of the dung are present in 
an insoluble condition and only become available as the 
organic matter decays. It is evident therefore that 
contrary to what is very generally believed by farmers 
the urine Or liquid excrement is much more valuable 
for manurial purposes than the dung or solid excrement. 

The proportion of water in urine is necessarily always 
large, ,but is extremely variable. It is usually over 
85 per cent., and sometimes as much as 98 per cent. It 
depends so largely upon the quantity of water drunk or 
taken in with the food, and the amount lost by per- 
spiration, that it can scarcely be said to be characteristic 
of the different kinds of animals. It may be said, how- 
ever, that, under normal and similar conditions, the urine 
of horses is generally more concentrated than that of 
cows ; also that the urine of sheep is generally more con- 
centrated, and of pigs more dilute than that of either of 
the former. 

The proportion and composition of the solid matter of 
the urine, like that of the dung, is affected by the charac- 
ter and quantity of the food supplied to the animal, but 
to an even larger extent. This may be illustrated by 
the example on p. 811, showing the quantities and com- 
position of dung and urine produced by cows fed on 
mangolds and lucerne hay respectively. In the first table 1 
the data are given as percentages of the fresh excrement, 
and in the second table as percentages of the dry matter. 
The third table shows the total quantities of the excreta, 
and the different'constituents of the same. 

It will be seen that both the dung and urine produced 
by the animals fed on lucerne hay contain a larger pro- 
portion of solid matter, but the most striking difference 

1 "Warington, " Chemistry of the Farm." 



FAEMYAED MANUEE 



311 



COMPOSITION OF THE DUNG AND URINE OF Cows FED ON MANGOLDS 
AND LUCERNE HAY. 

(1) PER CENT. OF THE FRESH EXCREMENT. 





Mangolds. 


Lucerne Hay. 




Dung. 


Urine. 


Dung. 


Urine. 


Water .... 


83-00 


95-940 


79-70 


88-230 


Nitrogen 


33 


124 


34 


1-540 


Phosphoric acid 


24 


Oil 


16 


006 


Potash .... 


14 


597 


23 


1-690 



(2) PER CENT. OF THE DRY MATTER. 






Mangolds. 


Lucerne Hay. 


Dung. 


Urine. 


Dung. 


Urine. 


Nitrogen 
Phosphoric acid 
Potash .... 


3-94 
1-41 

82 


3-10 
25 
14-90 


1-67 

78 
1-13 


13-08 
05 
14-36 



TOTAL QUANTITIES OF EXCRETA AND OF THE CONSTITUENTS OF THE 
SAME, PRODUCED BY COWS FED ON MANGOLDS AND LUCERNE HAY. 






Mangolds. 


Lucerne Hay. 


Dung. 


Urine. 


Total. 


Dung. 


Urine. 


Total. 


Water . . . 
Solids . 

Total excrement 


Lbs. 

34-86 
7-14 


Lbs. 

84-427 
3-573 


Lbs. 

119-287 
10-713 


Lbs. 
38-25 
9-75 


Lbs. 
12-35 
1-65 


Lbs. 

50-60 
11-40 


42-00 


88-000 


130-000 


48-00 


14-00 


62-00 


Nitrogen 
Phosphoric acid 
Potash . 


Lbs. 
14 
10 
06 


Lbs. 

113 
010 
525 


Lbs. 

253 
110 

585 


Lbs. 

16 
08 
11 


Lbs. 
216 
001 
237 


Lbs. 

376 
081 
347 



312 SOILS AND MANURES 

is in the proportion of nitrogen in the latter. The animals 
fed on mangolds produced more than six times as much 
urine as those fed on lucerne hay, and more than twice 
as much total excrement. The total quantity of nitrogen 
voided by the latter was nearly 50 per cent, greater, 
but the quantity of phosphoric acid was slightly less, 
and of potash considerably less than by the former. The 
differences observed bear a close relation to the composi- 
tion of the food and serve to illustrate its influence on the 
character of the manure. It may be observed, however, 
that a diet consisting wholly of roots is not a normal or 
generally appropriate one for cows. 

THE LITTER. 

The character of the litter employed affects the quality 
of the manure produced, according to its composition, i.e., 
the amount of fertilising ingredients it contains and con- 
tributes to the bulk, its power of absorbing the liquid 
excrement and its influence on the fermentation. 

Straw. The litter generally consists of straw, which 
is perhaps more suitable for the comfort and cleanliness 
of the animals than any other substance. Also it is 
produced on the farm, is not of much use for any other 
purpose, and its manurial value is greatly enhanced by 
being practically composted with the animal excreta. 
Ordinary cereal straws contain from 0*4 to 0*7 per cent, 
of nitrogen, 0'2 to 0'25 per cent, of phosphoric acid and 
from 1 to 1'5 per cent, of potash. The proportions 
of these ingredients are, however, very variable, and 
greater differences are often observed between two 
samples of one kind than between those of different 
kinds. Wheat straw is generally preferred to any of the 
others. Barley straw is disliked because of its dusty 
character. Oat straw is rather richer than any of 



FAEMYAED MANUEE 313 

the others, but it is also more valuable for feeding 
purposes. Pea and bean straws are much richer; the 
former contains nearly twice as much and the latter 
three times as much nitrogen as the cereal straws, but 
of course, they are not so plentiful. Dry straw usually 
contains about 15 per cent, of moisture, 5 per cent, of 
ash and 80 per cent, of organic matter. The last con- 
sists mainly of fibrous cellulose and does not decompose 
readily, but it has considerable power of absorbing 
liquids. 

Peat Moss. In grazing districts where many animals 
are kept, and often but little straw is produced, peat 
moss is largely used as a substitute. It is also employed 
in considerable quantities in town stables and cow- 
houses, where all the litter has to be purchased. Peat 
moss contains from about 1 to 2 per cent, of nitrogen, 
but is relatively very deficient in phosphoric acid and 
potash. The proportions of fertilising ingredients de- 
pend upon the amount of water, which, in commercial 
samples, is very variable. Average samples of peat 
moss, however, contain a larger proportion of nitrogen 
than average samples of straw, and produce a richer 
manure owing to its greater power of absorbing both 
liquids and gases. Peat moss can absorb from three 
to four times as much water as an equal weight of straw, 
and it does not so readily allow the escape of ammonia. 
On the other hand, it is a " short " material, and is not 
so cleanly and tidy for the animals as straw. Peat 
litter does not decompose so readily in the soil as straw, 
but the manure made with it being richer, it generally 
gives a somewhat better result. In cases where it is 
necessary to economise straw, a mixture of peat moss 
and straw is sometimes used, but is considered trouble- 
some in practice. A small quantity of moss spread in 
the wet places, however, helps to absorb much of the 



314 SOILS AND MANURES 

liquid which would otherwise escape and a considerable 
saving can often be effected in this way. 

Dried Leaves and Ferns. The composition of dried 
leaves and ferns has been given (p. 304). They are 
sometimes employed as litter, but have no great power 
of absorption and generally produce manure of some- 
what inferior quality, a large part of the most valuable 
fertilising matter being lost. 

Other Litters. Besides those mentioned above, a 
variety of other substances, e.g., sawdust, tannery 
refuse, and even ordinary loam ar.e sometimes used. 
Sawdust is a good absorbent, but decomposes very slowly, 
and is itself of little value. Loam is cheap and handy, 
but requires to be dried. It is heavy to handle, does 
not undergo fermentation, and is very dirty. Materials 
of this kind are only suitable for use in emergency. 

Quantity of Litter. In ordinary practice the quantity 
of litter is very roughly apportioned, yet it is evident 
that it must exercise a considerable influence on the 
character of the resulting manure. If the litter be 
stinted much of the liquid may escape absorption arid 
be lost. On the other hand, if excess is used the manure 
may be too dry, will be liable to loss during fermentation, 
and will be generally of inferior quality. There is some 
difference of opinion as to the most suitable quantity 
to be used under different circumstances. Ordinarily it 
varies from about 5 to 10 Ibs. per head per day, for horses 
and cattle. In view of the more watery character of the 
excreta, perhaps more should be allowed for the latter than 
for the former. 

FERMENTATION. 

The gradual conversion of the mixture of litter and 
animal excreta commonly called " long or fresh " manure 



FAKMYAKD MANUKE 315 

into the " short or rotten " condition is called " making " 
the manure. It is brought about by the action of micro- 
organisms, and involves a numerous and complex series 
of chemical changes which are collectively known as 
fermentation. Some of the changes are well understood, 
and are caused by known types of bacteria. Others are 
more obscure, can only be followed in outline, and are 
probably caused by many different types. The bacteria 
may be classified as aerobic and anaerobic, but some 
are indifferent and can act either in the presence or 
absence of air. The changes affect both the nitro- 
genous and the non-nitrogenous organic matter, and a 
considerable proportion usually about 25 per cent, of 
the dry matter disappears in the form of gases, and 
heat is generated. The temperature developed depends 
largely upon the rate at which fermentation takes place. 

Fermentation is at first of the aerobic type and takes 
place very rapidly, the action of the bacteria being 
favoured by the presence of air and of soluble carbo- 
hydrates in the straw. A considerable rise of tempera- 
ture is thus produced, but the carbohydrates are soon 
all consumed and the further entrance of air is hindered 
as the manure settles in the heap. The temperature is 
reduced again as the fermentation gradually changes to 
the anaerobic or putrefactive type and becomes much 
slower. 

Fermentation 'of the Non-Nitrogenous Matter. The 
non-nitrogenous matter of ordinary fresh manure consists 
mainly of cellulose and other carbohydrates which, in the 
presence of air, are oxidised, forming carbon dioxide and 
water as final products. This change can, however, 
only take place to a very limited extent as the supply 
of oxygen is soon used up. In the absence of air, carbon 
dioxide and water are also produced together with lactic, 
butyric, acetic and other similar acids, methane and 



316 SOILS AND MANURES 

free hydrogen. The peculiar organised structure of the 
straw vanishes, and what remains is humus (p. 109). 
The humic acid combines with the alkaline carbonates of 
ammonia and potash, liberated .by the fermentation, 
forming soluble compounds which impart the dark brown 
colour to the liquid that drains from manure heaps. 
This change is characteristic of the conversion from 
the fresh or long to the short condition. It does not 
necessarily involve any loss of fertilising ingredients, 
but on the contrary, by reducing the quantity of dry 
matter it tends to increase the proportion of fertilizing- 
constituents in the part that remains. 

Fermentation of the Nitrogenous Matter. The effects 
of the fermentation of the nitrogenous matter are not so 
readily apparent as those above described, but they are 
considerably more important. The nitrogenous matter 
of the urine consists of comparatively simple compounds 
and is probably the first to undergo those changes by. 
which ammonium carbonate is produced. The formation 
of this substance from urea takes place in two stages 
which may be represented by the following equations : 

(NH 2 ) 2 CO + H 2 = NH 4 .O.CO.NH 2 

Urea. Water. Ammonium 

carbamate. 

NH 4 .O.CO.NH 2 + H 2 = (NH 4 ) 2 C0 3 

Ammonium Water. Ammonium 

carbamate. carbonate. 

It will be seen that both reactions consist in the addi- 
tion of the elements of water; they proceed simul- 
taneously and may be represented by a single equation. 
The change can be brought about by a number of different 
organisms and probably takes place either in the presence 
or absence of oxygen without evolution of nitrogen. 

The nitrogenous compounds of the dung are, for the 



FAEMYAED MANUEE 317 

most part, insoluble and more complex than those of the 
urine, and they undergo fermentation much more slowly. 
They consist mainly of albuminoids, the decomposition 
of which by bacterial action has been previously alluded 
to. It was shown (p. 154) that they pass through a 
series of complex changes, the nitrogen being converted 
into amino-acid bodies and finally into ammonia. 

The nitrogenous matter of the litter ultimately suffers 
a like fate, the change taking place more or less readily 
according to the nature of the substance. In wheat 
straw about a fifth part oi the nitrogenous matter is 
digestible, and it is to be supposed that this part is 
more readily attacked by the microbes than the remainder, 
and also more readily than the nitrogenous matter of the 
dung, which, it will be remembered, consists of the 
indigestible portions of the food. 

The Fate of the Ammonia. The conversion of the 
nitrogen into ammonium carbonate may be regarded as 
the completion of the first stage, but it is subject to 
further changes which materially affect the character of 
the manure. A certain amount of ammonium compounds 
is nearly always present as they are continually being 
formed, but they do not accumulate as might be expected. 
The largest amount is produced during the initial stages 
of fermentation ; the quantity rises to a maximum and 
then diminishes again. The longer the manure is kept 
the smaller is the proportion of ammonium compounds 
generally found. It is obviously a matter of importance 
to follow them up and see what becomes of them. 

Under certain conditions which are not well defined, 
some of the nitrogen is liberated in the free state. 
The phenomenon is called denitrification, but it is not 
clear whether it is due to the formation and subse- 
quent reduction of nitric acid or to the direct oxidation 
of the hydrogen of the ammonia. In either case it can 



318 SOILS AND MANURES 

only take place in the presence of air. Free nitrogen 
may possibly be produced in both ways, but in farmyard 
manure the conditions are not generally favourable to the 
nitric fermentation (p. 157). It is evident that whatever 
the cause, the nitrogen liberated in the free state is lost 
beyond all hope of recovery. 

A more potent cause of the disappearance of ammonium 
compounds under ordinary circumstances, is their re- 
conversion into protein compounds by the action of cer- 
tain bacteria which can utilise the nitrogen for the for- 
mation of the albuminoids of which they are partly 
composed. This change, it will be seen, is exactly the 
reverse of those previously described. It results in the 
conversion of the soluble nitrogenous matter into in- 
soluble forms. It tends to conserve the nitrogen, but 
reduces its manurial activity. The two kinds of change 
probably occur simultaneously, and the amount of solu- 
ble nitrogenous matter present at any time, depends upon 
the relative rates at which they take place. The forma- 
tion of ammonium carbonate takes place at first very 
rapidly but afterwards becomes slower and is gradually 
overtaken by the reverse change. 

The soluble nitrogenous matter is also liable to direct 
loss by volatilisation of ammonia and by solution in 
water which drains from the heap. 

Loss IN MAKING AND STORING. 

The loss of fertilising ingredients which farmyard 
manure suffers during the process of making .and storing, 
arise principally from the causes mentioned above, viz., 
the evolution of free nitrogen gas, volatilisation of am- 
monia and the drainage of water. In actual practice 
the losses cannot be altogether prevented, but they can 
be minimised by careful management. 



FAEMYAED MANUEE 319 

Loss by drainage. When fresh manure is removed 
from the stables and cowhouses it is often nearly satu- 
rated ; as fermentation proceeds more water is formed 
and the organic matter by which it is retained is gradu- 
ally diminished in quantity. In addition to this, the 
manure is often exposed to the weather and rain soaks 
into it. The excess of water gradually niters down- 
wards and, if allowed to escape, all the fertilising ingre- 
dients which it holds in solution, i.e., the most active 
and valuable part of the manure, are lost. The manure 
should not, therefore, be placed on an eminence or 
sloping place as is sometimes done, for in such positions 
the liquid rapidly drains away. 

In order to prevent the waste of manure in this way, a 
special receptacle is constructed in the yard on many of 
the larger farms. It consists of a shallow pit or saucer- 
shaped depression, the bottom of which is covered with 
cement or other watertight material, laid with a fall 
towards the centre so that all the liquid collects in a 
tank placed there to receive it. A pump is fitted in 
connection with the tank so that the liquid can ( be 
pumped up and distributed over the heap from time to 
time. A fairly good substitute can he made by simply 
scooping out a few inches of soil and covering the bottom 
with a layer of plastic clay two or three inches thick and 
well beaten in so as to render it impermeable to the 
liquid. The pit may be round or square, but should 
be deeper towards the middle than at the sides. The 
earth which is dug out may be used to form a lip or 
bank round the margin so as to prevent the access of 
surface water. This simple arrangement can be carried 
out at the cost of a little trouble which it amply repays. 

Similar precautions should be observed in the con- 
struction of fold-yards and boxes. The bottoms should 
be below the level of the ground, and should be 



320 SOILS AND MANUBES 

covered with cement or clay. When the manure is 
to be stored for a considerable time it is often built up 
in the form of a mound with sloping sides, like the roof 
of a house, and thatched over to protect it from rain. 
This can be done as the manure accumulates, and is not 
only cheaper but is considered better than a built roof, 
which in summer at least tends to promote loss by 
evaporation. As a special precaution, a layer of peat is 
sometimes placed under the manure to absorb the liquid, 
which instead of being lost thus forms an additional 
quantity of valuable manure. It is to be noticed that the 
loss by drainage affects not only the nitrogen but all 
the soluble constituents more or less alike. 

Loss by Evaporation. A certain loss of nitrogen by 
evaporation of ammonia and the liberation of the free 
element is practically unavoidable. In both cases it is 
due mainly to the access of air, and is therefore greatest 
when the manure is in a loose state. The liberation of 
free nitrogen gas, it has been shown, results directly or 
indirectly from the oxidation of ammonia. The two 
factors which chiefly govern the loss by evaporation of 
ammonia are the temperature and the extent of surface 
exposed. The high temperature produced by the more rapid 
fermentation which takes place in the presence of air, 
causes the ammonium carbonate to dissociate into am- 
monia and carbonic acid, and a loose condition of the 
manure also facilitates the escape of the gases. 

It has been shown that fermentation is most rapid 
and the temperature highest during the initial stages, 
and it is probably during this stage that the greatest 
loss occurs. As the manure becomes more compact the 
rate of fermentation falls off, owing to the exclusion of 
air, and the temperature is gradually reduced. Under 
these conditions the loss of nitrogen both in the free 
state and in the form of ammonia is greatly diminished. 



FARMYARD MANURE 321 

When the manure is deposited in small loose heaps, the 
amount of loss is very great. They never become con- 
solidated and rapid fermentation continues until arrested 
by desiccation. Manure which has been allowed to 
become dry and mouldy in this way is generally of very 
inferior quality. 

These observations are of great practical interest as 
showing the importance of consolidating the manure in 
the process of making, and of leaving it undisturbed 
until it is carried out to the land. If it is turned or 
moved, active aerobic fermentation is resumed and loss 
of nitrogen results. It has been shown by experiments 
at Woburn and elsewhere that the least loss occurs in 
actual practice when the manure is made in yards or 
boxes. These are constructed to prevent any loss by 
drainage ; the manure is well trampled by the cattle 
and never disturbed, fresh litter, being spread on the 
top of the old as required. Under these conditions the 
loss of nitrogen may be reduced to from 10 to 20 per 
cent, of the total amount in the food, in addition to what 
is retained by the animals. When the manure is carried 
out day by day and made into' a heap in the yard, with 
the most careful management, the loss is at least double 
that amount and may be very much more. In the, 
Woburn experiments the loss in making the manure in 
boxes as above described amounted to from 13 to 18 per 
cent, of the total nitrogen ; the manure was afterwards 
removed and stored in a heap during the winter, when a 
further loss of about 20 per cent, took place. In 
Maercker and Schneidewind's experiments the manure 
made in boxes and well trampled by the animals, lost 
13 per cent, of nitrogen, whereas that carried out day 
by day to a heap in the yard lost 37 per cent., care 
being taken to prevent any loss by drainage. The 
manure made an the boxes was allowed to remain there 

S.M. Y 



322 SOILS AND MANUKES 

for a month after the animals were removed, and the loss 
increased to 35 per cent., the weather at the time being 
very warm. 

In ordinary farming practice the losses are generally 
much greater than those recorded in the experiments re- 
ferred to. Many of the animals are kept in stalls and 
there is often a considerable waste of liquid excrement. 
The manure is commonly carried out daily, and little 
care is exercised to prevent loss by evaporation or by 
drainage from the heap. When the amount retained by 
the animals and all sources of loss are taken into 
consideration, it is probable that, of the total nitrogen in 
the food, only about from one-half to a quarter is re- 
covered in the manure. The losses in all these ways fall 
most heavily upon the soluble, i.e., the most active and 
valuable part of the manure. 

AMMONIA FIXERS AND PRESERVATIVES. 

Gypmm, etc. A quantity of powdered gypsum is some- 
times mixed with the manure in order to check the loss of 
nitrogen by evaporation of ammonia. Its action is due to the 
chemical change represented by the following equation : 

CaS0 4 + (NH 4 ) 2 C0 3 = CaC0 3 + (NH 4 ) 2 S0 4 . 

The efficiency depends to some extent upon the quantity 
of the preservative used, and the saving effected by this 
means is not commensurate with the cost of the material. 

Sulphate of magnesia and sulphate of iron (copperas) 
have also been proposed as substitutes for gypsum. 
They act in a similar manner but are more soluble 
and probably more efficient than gypsum. 

Acid substances, e.g., sodium .bisulphate, superphos- 
phate and even sulphuric acid itself have been tried and 
have proved more effective than the neutral sulphates. 
The best results were obtained by the use of superphos- 



FAEMYAED MANUEE 323 

phate, which has the further advantage that it fortifies 
the manure in the constituent (phosphoric acid) in which 
it is most deficient. On the other hand, it deteriorates 
the manurial value of the superphosphate. The phos- 
phate is precipitated, and though the compound is readily 
available to plants, it probably does not .become so finely 
mixed with the soil, which is the peculiar advantage of 
superphosphate. Acid substances should not be mixed 
with -manure that is to be trodden by animals, as they 
have an injurious effect upon their hoofs. Sulphates, 
whether neutral or acid, are objectionable on another 
ground; they are easily reduced to sulphides which, in 
large quantity, are deleterious to the crops. 

Instead of sulphates, hydrochloric acid and neutral 
chlorides have been used. Of these, kainite has been, per- 
haps, the most largely employed. It is open to the objec- 
tion, however, that farmyard manure does not generally 
require the addition of potash, and it is not profitable to 
use it solely for the saving of nitrogen which it effects. 
Besides, chlorides, like sulphides, have a deleterious 
effect on vegetation. 

Antiseptics. More recently it has been proposed to 
treat the manure with antiseptics in order to arrest or 
retard fermentation. These are generally more expen- 
sive and not more effective than the simple ammonia 
fixers previously mentioned. If they were effective their 
use must be unsparingly condemned because, in that 
case, the straw would not be converted into humus and 
the insoluble plant "foods would not be rendered avail- 
able. Volatile substances which are transient in their 
effects and only delay fermentation are not so objection- 
able, but they are of very little practical use. One of 
the objections to the use of kainite as an ammonia fixer 
is that when used in sufficient quantity, the antiseptic 
action of the chlorides prevents the manure from rotting. 

Y 2 



324 SOILS AND MANUEES 

Superphosphate also, it is ^said, has a similar effect but 
not so marked as that of kainite. 

Absorptives. The well-known power of peat to absorb 
ammonia gas has indicated this substance as likely to 
prove useful as a means of preventing loss. It has been 
tried in the ordinary condition and also impregnated with 
sulphuric acid but without much success. Perhaps a 
quantity of ordinary loamy or peaty soil might be used 
with advantage. It costs nothing, does not interfere with 
the fermentation, and at the worst could do no harm. 
It would probably prove as efficient as those ammonia 
fixers which depend upon chemical action. 

COMPOSITION. 

The composition of farmyard manure, it will thus be 
seen, depends upon a number of variable factors, and 
the differences in quality are sometimes very great. 
Practical farmers have always recognised the difference 
between horse and cow manure, fresh and rotten manure, 
and they have long been familiar with the improve- 
ment produced by feeding the animals on cake and other 
rich food. They do not, however, as a rule, attach so 
much importance to the conditions under which the 
manure is made. Under ordinary circumstances, it is 
perhaps surprising that the variation in composition is 
not greater than is commonly found. Nevertheless, in 
the case of mixed manure made with straw litter in the 
ordinary way, one sample may contain twice as much 
fertilising matter as another. The proportion of water 
depends upon accidental circumstances and is very vari- 
able; as it affects the proportions of all the other ingre- 
dients, these should be given as percentages of the dry 
matter, which is the only proper basis for comparison. 
Unless this is properly understood analyses of farmyard 



FAEMYAED MANUEE 



325 



manure are apt to prove misleading. Any statement 
of limits or averages must therefore be accepted with 
considerable reserve. For example, it may be said that 
average samples of mixed manure contain from 0'5 to 
0*75 per cent, of nitrogen, from 0'2 to 0'3 per cent, of 
phosphoric acid and from 0'3 to 0'4 per cent, of potash. 
These, however, are not the limits of variation. Some 
samples are richer and others are of much poorer quality 
and such are by no means uncommon. 

For any ingredient O'l per cent, is equal to very 
nearly 2J Ibs. per ton. According to Warington, l 
average samples contain from 9 to 15 Ibs. of nitrogen, a 
similar quantity of potash and from 4 to 9 Ibs. of phos- 
phoric acid per ton. Calculating from th>e mean of the 
figures given above, each ton of manure should contain 
about 14 Ibs. of nitrogen, 8 Ibs. of potash and 5| Ibs. of 
phosphoric acid. 

The following analyses may be taken as typical, and 
serve to illustrate some of the points referred to : 

ANALYSIS OF MIXED MANURE, FRESH AND ROTTED (VOELCKER). 






Fresh. 


Rotted. 




Per cent. 


Per cent. 


Water 


66-17 


75-42 


Organic matter (soluble) 1 . 
,, (insoluble) a 


2-48 ) 9ft 94 
25-76 ) 


3-71 

12-82 


16-53 


Ash (soluble) 3 . 


1-54 ) _ - Q 


1-47 


8-0^ 


,, (insoluble) 4 . 


4-05 j ' 


6-58 






100-00 


100-00 


1 Containing nitrogen 

2 


149) ..o 
494) c 


297 
309 


606 


8 tricalcic phosphate '080 ) . 


\ .^ 


4 ,, -275 j 


485 




, potash . 


158 ) . 9 o n 


082 


119 


4 


072) 


037 





Chemistry of the Farm." 



326 



SOILS AND MANURES 



COMPOSITION OF LONDON STABLE MANURE (DYER). 









Mixed Peat and Straw Litter. 





Peat 
Moss. 


Stiaw. 


Fresh. 


After Storage. 








1. 


2. 


1. 


2. 


3. 




Per 


Per 


Per 


Per 


Pei- 


Per 


Per 




cent. 


cent. cent. 


cent. 


cent. 


cent. 


cent. 


Water , 


77-8 70-0 ! 76-1 


62-0 


53-8 61-9 


52-9 


Organic matter 


18-0 ; 24-3 19'3 


26-4 


17-5 


22-0 


23-0 


Nitrogen (soluble) . O51 O52 O08 


0-08 


0-06 


0-08 


o-io 


(insoluble) 0'37 O'lO O46 


0-62 


0-58 0-68 


0-79 


Phosphoric acid . 0'37 0'48 


0-33 


0-45 


0-49 0-56 


0-66 


Potash . 


1-02 


0-59 


0-45 


0-58 


0-58 


0-65 


0-80 



COMPOSITION OF HORSE AND Cow MANURE (STORER). 






Horse Manure. 


Cow Manure. 


1. 


2. 


3. 


1. 


2. 


3. 




Per 


Per 


Per 


Per 


PIT 


Per 




cent. 


cent. 


cent. 


cent. 


cent. 


cent. 


Water. 


75-76 


67-23 


71-15 


85-30 


72-87 


77-06 


Dry matter. 


24-24 


32-72 


27-67 


14-70 


27-13 


22-93 


Ash .... 


5-07 


6-49 


4-65 


2-04 


6-70 


4-30 


Potash 


0-51 


0-22 


0-49 


0-36 


1-69 


0-64 


Lime .... 


0-30 


0-17 0-36 


0-29 


0-41 


0-48 


Magnesia . 


0-19 


0-20 0-20 


0-19 








Phosphoric acid . 


0-41 


0-35 j 0-36 


0-16 


0-20 


0-14 


Ammonia . 


0-26 


0-15 


0-24 


0-06 





0-14 


Total nitrogen 


0-53 


0-47 


0-59 


0-38 


0-79 


0-48 



MANURIAL VALUE. 

It is unnecessary to magnify the fertilising value of 
farmyard manure. It cannot properly be compared with 
that of special manures, though this has sometimes 
teen done to the disadvantage of the latter. The 
manurial value of farmyard manure depends chiefly 
upon its mechanical effects, the amounts of fertilising 



FARMYARD MANURE 327 

ingredients it contains and the state of combination in 
which they are present. 

Mechanical Effects. The characteristic and highly 
beneficial effects of farmyard manure upon the texture 
of soils are due to the organic matter, of which it ordin- 
arily contains about 20 per cent, in the wet condition. 
It helps to floculate clay soils and reduce them to a state 
of tilth which in some cases cannot be produced hy any 
other means. On sandy soils it has just the opposite 
effect ; it binds them together and greatly increases their 
power "of retaining moisture. Its effects are, for this 
reason, often seen to greatest advantage in droughty 
seasons, which are in other respects the least favourable 
to its action. The mechanical effects of farmyard manure 
are perhaps not less important than those which depend 
upon its chemical composition, and, on this account, 
it is considered by many to be practically indispensable. 
If it is not produced or cannot be obtained in sufficient 
quantity to maintain the necessary supply of humus in 
the soil, recourse must be had to green manures, and 
the supply of plant foods can be supplemented by special 
concentrated fertilisers. 

The Fertilising Ingredients. The fertilising effects of 
farmyard manure are particularly striking when regarded 
from the point of view of its chemical composition. It 
is a general manure containing nitrogen, phosphoric 
acid and potash, but the proportions of these ingredients 
are small generally less than one per cent, of each. If 
the manure were used in handfuls or hundredweights the 
quantities of these constituents would indeed be insig- 
nificant, but when the quantities commonly applied are 
taken into account, the matter assumes a different aspect. 
An ordinary dressing of say 10 tons per acre of good 
manure would supply about 150 Ibs. of nitrogen, 100 Ibs. 
of potash and 60 Ibs. of phosphoric acid, To obtain 



328 SOILS AND MANURES 

these quantities in the form of artificial manures, it 
would be necessary to employ, in round numbers, about 
9 cwts. of nitrate of soda, 8 cwts. of kainite and 5 cwts. 
of superphosphate. Apart from the question of expense, 
it is obvious that such quantities could not be employed 
without seriously deteriorating both the soil and the 
crop to which they might be .applied. The quantity of 
nitrate is about j^our times as much as is commonly applied 
at one time ; if it were divided into four, and one part 
applied each year, the allowance would be regarded as 
liberal, and large returns might be expected. This, how- 
ever, does not hold good equally for the phosphates and 
kainite. 

It will thus be seen that an ordinary dressing of farm- 
yard manure supplies large amounts of plant food, and 
the manurial effects produced by it are attributable, in 
great measure, to this fact. 

State of Combination. It is frequently claimed for 
farmyard manure that it is both rapid in its action and 
lasting in character. It can be applied in large quanti- 
ties at a time without injury to the crops and with- 
out much risk of loss of plant foods. It may be 
assumed therefore that the state of combination in 
which the fertilising ingredients, and particularly the 
nitrogen, are present, is different from that of the nitrogen 
in nitrate of soda or in the slower acting nitrogenous 
manures, such as bone meal, etc. It is evident from 
what has been said regarding the complex nature of the 
materials of which the manure is composed, and the fer- 
mentative changes which they undergo in the process of 
making, that this is so. It is evident, in fact, that the 
nitrogen in farmyard manure exists not in one, but in 
several, perhaps in many, different forms. 

Part of the nitrogenous matter is soluble in water ; it 
readily undergoes nitrification, and is immediately avail- 



FAEMYAED MANCJEE 329 

able for the plants. Assuming that the proportion of 
soluble nitrogen is 0*1 per cent., 10 tons of manure would 
contain 22 Ibs., a quantity which is equal to 1 cwt. of 
sulphate of ammonia and is sufficient to account for the 
immediate and rapid action. The proportion of soluble 
nitrogenous matter is always small because it is recon- 
verted by bacterial action into insoluble protein com- 
pounds which, though not immediately available, prob- 
ably hecome so very readily. At all events they are 
probably much more easily converted into a soluble 
state than the original albuminoids of the dung and 
litter. It is difficult to say what proportion of the nitrogen 
may be present in this form. It varies according to the 
length of time and the conditions under which the manure 
has been kept. It is perhaps the most valuable part of 
the manure. Being insoluble it is not liable to loss, yet 
it is readily available to the plants. It comes into action 
gradually, thus keeping up a constant supply through- 
out the period of growth, but is never excessive at any 
one time. The fertilising ingredients of the unfermented 
portions of the dung and litter are more slowly available, 
and it is to these that the manure owes its lasting char- 
acter. They come into action as the organic matter is 
slowly oxidised in the soil, and thus a constant but' 
gradually diminishing supply of available plant foods is 
maintained, sometimes for many years. The fertilising 
ingredients in tjiis part of the manure probably exist 
in many different degrees of availability according to the 
nature of the materials. The length of time that the 
manure will last, depends upon the rate at which the 
plant foods are converted to the available state, and 
therefore to some extent upon the character of the soil. 
On open sandy soils the organic matter is more rapidly 
oxidised and a larger proportion of the fertilising ingre.- 
dients comes into action each year. The lasting character 



330 SOILS AND MANURES 

of the manure is generally much appreciated by farmers, 
but it is clear that the part of the manure to which it 
is due is economically the least valuable. The more 
rapidly the fertilising ingredients of a manure are re- 
covered in the crops, the more frequently they can be 
used again. 

APPLICATION. 

When manure is carried out to the land it should be 
spread and ploughed in as quickly as possible. It is 
sometimes left for a considerable time in small heaps or 
spread out in a thin layer on the surface, but these are 
the very conditions under which the greatest amount of 
loss occurs and should be avoided as far as possible. The 
same objection applies to the use of farmyard manure 
for grass land ; it is not covered over arid a considerable 
amount of nitrogen is lost by evaporation during the 
process of decay. On pastures, of course, it is not re- 
quired ; the growth of the grasses accumulates a large 
supply of humus in the soil and the plant food is largely 
restored in the droppings of the animals. In regard to 
meadows and rotation grass there are other points to be 
considered. They often stand in need both of humus and 
of plant foods. Of the beneficial effects of farmyard 
manure in such cases there is no room for doubt. The 
only question is as to what is the best use that can be 
made of it. 

There is a general consensus of opinion that, in ordin- 
ary mixed farming, the best results are obtained by 
applying the manure to the root crops and using arti- 
ficials for the grass land as required. In some districts, 
however, exactly the reverse course 'is pursued ; prac- 
tically the whole of the farmyard manure is reserved 
for the grass, and the arable land is cultivated mainly 
with special manures. There is one great advantage in 



FARMYAKD MANURE 331 

this method, viz., the arable land is kept much cleaner. 
One of the arguments used in favour of applying the 
manure to the green crops is that it affords a favourable 
opportunity of eliminating the weeds. This work has, 
of course, to be done in any case, but it is all the easier 
when no fresh weeds are introduced in the manure. 

The customary practice in most parts of the country 
is to store the manure for some time and apply it in the 
short, well rotted condition, but in some cases it is 
applied in the long or fresh state. When applied in the 
fresh state there is probably less risk of loss by evapora- 
tion of ammonia, but more is lost in the form of free 
nitrogen and it has been found to set up denitrification 
in the nitrates of the soil. For the production of tilth 
in clay soils, fresh manure is probably the more effica- 
cious and should be applied in the autumn. For light 
sandy soils which require cohesion, well-rotted manure 
applied in the spring is undoubtedly the best. 



APPENDIX I 



INSTRUCTIONS FOR VALUING MANURES 

Issued by the Highland and Agricultural Society of 
Scotland. 

THE unit used for the valuation of manures is the hundredth part 
of a ton, and as the analyses of manures are expressed in parts per 
hundred, the percentage of any ingredient of a manure when multi- 
plied by the price of the unit of that ingredient represents the value 
of the quantity of it contained in a ton. 

As an example take muriate of potash : a good sample will be 
guaranteed to contain 80 per cent, pure muriate of potash, the 
other 20 per cent, consisting of unimportant impurities, such as 
common salt. But all potash manures are valued according to the 
amount of potash they yield, and 80 per cent, of pure muriate of 
potash yields 50 per cent, potash (K 2 0) i.e., 50 units per ton ; and as 
a ton of muriate of potash costs 8 15s., the price of the unit is the 
fiftieth part of that, viz., 3s. 6d. If on analysis, a sample of muriate 
of potash, guaranteed to contain 50 per cent, of potash, is found to 
contain only 49 per cent., the price per ton will be 3s. Qd. less, viz., 
$ 11s. Qd. 

Similarly with all other manures, the price per unit is derived from 
the price per ton of a sample of good material up to its guarantee, 
and therefore the proper price per ton of a manure is found by 
multiplying the price per unit of the valuable ingredient by the 
percentage as found by analysis. If a manure contains more than 
one valuable ingredient, the unit value of each ingredient is multiplied 
by its percentage, and the values so found when added together give 
approximately the price per ton of the manure. 

Nitrate of soda contains no ammonia, but it contains nitrogen, and 
14 units of nitrogen are equivalent to 17 units of ammonia. 



334 APPENDIX I 

The commercial values of manures are determined by means of the 
units in the following manner : 

Take the analysis of the manure, and look for the following 
substances : 

Phosphates dissolved (or soluble phosphate) ] 

Phosphates undissolved (or insoluble phos- / No other items but 

phate) . . . . > these are to be 

Nitrogen ( valued. 

Potash . . . . . . . . ) 

Should the analysis or the guarantee not be expressed in that way, 
the chemist or the seller should be asked to state the quantities in 
these terms. 

Suppose the manure is bone meal : 

An ordinary bone meal will contain about 50 per cent, phosphate 
and about 3f per cent, nitrogen. The units for bone meal are 
Is. 3d. for phosphate and 12s. Id. for nitrogen. Therefore the 
value is 

s. d. 
Insoluble phosphate, 50 times Is. 3d., 

equal to 326 

Nitrogen, 3| times 12s. Id., equal to .254 

Sa,y 5 1 10 per ton. 

Suppose the manure is dissolved or vitriolated bones : 
It must be guaranteed *' pure." 
The units in the schedule are 2s. 3d. for soluble phosphate, Is. 8d. 

for insoluble phosphate, and 15s. Qd. for nitrogen. 
The analysis will be about 16 per cent, soluble phosphate, 18 per 
cent, insoluble phosphate, and 2| per cent, nitrogen. In that 
case the value would be 

s. d. 

Soluble phosphate, 16 times 2s. 3d., equal to 1 16 
Insoluble 18 Is.Sd., 1 10 
Nitrogen, 2| times 15s. Qd., equal to . .228 

Say .5 8 8 per ton. 

Suppose the manure is a superphosphate say an ordinary super- 
phosphate, with 38 per cent, soluble phosphate and 2 per cent, 
insoluble phosphate. It is valued thus 



APPENDIX I 



335 



s. d. 
Soluble phosphate, 38 times Is. 11^., equal 

to say 3 12 10 per ton. 

Insoluble phosphate is not valued in a superphosphate. 

NOTE. The units have reference solely to the MARKET PRICE of 
manures, and not to their AGRICULTURAL VALUES. 

Thus, in stating soluble phosphate in dissolved bones at 2s. 3d. per 
unit, and that in superphosphate at Is. 11^., it is meant that these 
are the prices per unit at which soluble phosphate can be bought in 
these two manures ; but it does not mean that the soluble phosphate 
in the one is d. per unit better as a manure than that in the other. 
It is probably no better. 



UNITS TO BE USED IN DETERMINING THE MARKET PRICE OF MANURES. 
Terms Cash, including bags gross weight not including carriage. 

N.B. These units are based on the retail cash prices of manures at 
Leith and Glasgow. When these units are multiplied by the 
percentages in the analysis of a manure, they will produce a value 
representing very nearly the cash price per ton at which two tons 
may be bought in fine sowable condition at Leith or Glasgow. 
Larger purchases may be made on more favourable terms, but 
for smaller purchases an extra charge of Is. Qd. per ton is made. 

FOR SEASON 1909. CASH PRICES AS FIXED ON 3RD FEBRUARY. 





Peruvian (Riddled). 








Nitrogenous. 


Phosphatic. 




Bone Meal. 






Per unit. 


Per unit. 


Per unit. 


Per unit. 






s. d. 


s. d. 


s. d. 


s. d. 


Phosphates dissolved . 1 ) .. . 
,, undissolved ' j 


1 4 


1 4 


1 3 


Potash . 


. 36 


3 6 








Nitrogen 





16 


15 


14 


12 1 


Prices per ton 


| From 
j to 


120 
upwards 


95 
upwards 


95 

upwards 


105 
115 



336 



APPENDIX I 











Superphosphates. 


Items to be Valued. 


Steamed 
Bone Flour. 


Dissolved or 
Vitriolated 
Bones. 




Under 30 per 
cent. Sol. 


30 per cent. 
Sol. or over. 






Per unit. 


Per unit. 


Per unit. 


Per unit. 






s. d. 


s. '/. 


s. d. 


<l. 


Phosphates 

? 


dissolved . 
undissolved 


1 2 


2 3 
1 8 


1 11 


1 11 


Potash . 


! 











Nitrogen 


. 


12 1 


15 6 








( From 
Prices per ton j , 


H5 
95 


105 
110 


50s. 
73s. 



MANURES. 



At Leith and Glasgow, except in case 
of Thomas-Slag Phosphate. 


Guarantee. 


Price per ton. 


Unit. 




Per cent. 


t s. d. 


st. d. 


Sulphate of ammonia 


20 Nitrogen 


11 15 


Nit. =11 9 


Nitrate of soda, 95 per cent. 


15-5 


9 10 


=12 3 


Muriate of potash, 80 per 






cent. . . ... 


50 Potash 8 15 


Pot. = 36 


Sulphate of potash 


52 


9 15 


,,=39 


Kainit (unpulverised) . 


12-4 250 


= 3 73 


Potash salts 


30 ., 4 15 


,,=32 


Basic slag (Thomas-phos- < 


22 Phosphate 176 


Phos.= l 3 


phate powder), at place j 


30 


1 15 


=12 


of production . . ( 


38 


250 


,,=12 


Ground mineral phosphate 


60 


2 10 


= 10 



Peruvian guano 



CLASSIFICATION OF MANURES. 



Guanos with over 4 per cent, of nitrogen are to 
be considered as nitrogenous. Those with 
less than this percentage are to be classed as 
I phosphatic guano. 



APPENDIX I 



337 



Bone meal 



Steamed bone flour 



Dissolved bones 



Mixtures and com- 
pound manures 



Basic slag (Thomas 
phosphate powder) 



Genuine bone meal contains from 48 per cent, 
to 55 per cent, phosphates, and from 3| per 
cent, to 4^ per cent, nitrogen. If phosphates 
are low nitrogen will be high, and conversely. 
If bone meal is so finely ground that 90 per 
cent, or over passes a sieve of ^-inch mesh, 
an addition of 2s. Qd. per ton should be made 
to the valuation. 

, Ground to flour, and containing about 60 to 65 
per cent, phosphates and about 1 to 1^ per 
cent, nitrogen. 

JMust be pure i.e., containing nothing but 

\ bones and sulphuric acid. 

fTo be valued according to the following unit- 
prices : nitrogen, 12s. ; soluble phosphate, 
Is. 11^. ; insoluble phosphate, Is. 2d. ; potash, 
3s. 3d. ; with an addition of 4s. per ton for 
bags and 7s. Qd. per ton for mixing. These 
units give the cash price at Leith and Glas- 
gow. They apply only to mixtures made 
from high-class materials. For instance, the 
nitrogen of mixtures valued by these units 
should not be from shoddy, hair or leather, 
or the insoluble phosphates from ground 
mineral phosphates. 

About 90 per cent, of the phosphate should be 
citric soluble (official method of Board of 
Agriculture). 

Fineness of grinding is of importance. The 
coarsest kind used should be so finely ground 
that at least 80 per cent, passes through a 
wire sieve of about 9,600 holes per square inch. 



S.M. 



APPENDIX II 



COMPOSITION AND MANURIAL VALUE OF VARIOUS 
FARM FOODS. 

(CALCULATED BY DR. CHARLES CROWTHER.) 

GREAT variations occur in the composition of any particular food, 
but the following data have been compiled from a number of sources, 
and must be considered as having reference, in each case, to the 
average composition of the food. 

MANORIAL INGREDIENTS. 





Per Cent. 


Per Ton. 


|l|s|| 


Food. 






^ 


j-5 




O 'o 


Q" 




||!p|^| 




a 

* 


'o 


tt 





d 


' 


S 

w 


i 


^ g J -e"! s 




I 


'^A 


"3 





It 

E 


^8i 


& 


0, 

2 


||^|cp|f 







-C 32 





B 


3 


2 ^ 


s 




^ J3 *> I .H ^ O 




^ 


PH ^ 


o 


13 


K 


QJ *o 




.n 


C'wO o ^ '~ l '^'a^ 






<j^ 


p-l 


*""* 




<5 


1^; 


H 3 


- r^ p "> ^ 2 "^ o 




















^ E'S 3,5 "En's t> 




Per 


Pei- 


Pei- 


Per 














cent. 


cent. 


cent. 


cent. 


Lb. 


Lb. 


Lb. 


Lb. 


6'. (?. 


Cottonseed cake 




















decorticated 


6-9 


3-1 


1-6 


0-3 


155 


70 


36 


7 


2 14 10 


Cottonseed cake 




















undecorticated 


3-6 


2-5 


1-6 


3 


80 


56 


36 


7 


1 13 7 


Linseed cake . 


4-7 


1-7 


1-3 


4 


106 


38 


29 


9 


1 17 3 


Rape cake 


4-9 


2-0 


1-3 


7 


110 


45 


29 


15 


1 19 1 


Earthnut cake . 


7'5 


1-3 


1-5 


2 


168 


29 


33 


4 


2 14 


Cocoanut cake . 


3'4 i 1-5 


2-0 


5 


76 


33 


44 


11 


1 11 10 


Palm-nut cake 


2-6 


1-1 


5 


3 


58 


24 


11 


7 


101 


Linseed . 


3-6 


1-4 


1-1 


3 


86 


32 


24 


7 


192 


Locust beans . 


1-0 


5 


7 


? 


23 


11 


15 


p 


9 11 



For references, see p. 340. 



APPENDIX II 839 

MANURIAL INGREDIENTS continued. 





Per Cent 


Per Ton. 


3 .2 ^ ^ * ^ 








1^*4 g pLi ^ ^ 




















0) Q C|^ ^ O ^ ^ 


Food. 




O "^ 


^ 






. - i. 


^ 




r* rj Q +5 05 ^ /-v^ 




| 


? 


w 


t 


1 


If 





5" 

__ce 


3^||s|f 




o 


A s "^' 


i 


* 


6" 


P*""-'' 


_ 


*-s 


*2 *2 k ^ <j rR "** 




fc 


H 


ID 


a 


g 


11 


'I 


<X> 

a 






Per 


Per 


Per- 


Per 














cent. 


cent. 


cent. 


cent. 


Lb. 


Lb. 


Lb. 


Lb. 


s. d. 


Wheat middlings 




















(fine pollards) 


2-4 


1-4 


8 


05 


54 


31 


18 


1 


109 


Wheat sharps 




















(coarse pollards) . 


2-5 


2-6 


1-4 


1 


56 


58 


31 


2 


166 


Wheat bran 


2-4 


2-7 


1-5 


2 


54 


60 


33 


4 


166 


Oatmeal . 


2-4 


2-4 


1-5 


1 


54 


54 


33 


2 


1 5 10 


Maize germ meal . 


2-3 


8 


5 


1 


52 


18 


11 


2 


17 7 


Gluten meal 


61 


3 


05 


05 


136 


7 


1 


1 


1 17 6 


,, feed 


4-2 


7 


2 


1 


94 


15 


4 


2 


177 


Rice meal 


1-9 


2-5 


7 


1 


42 


56 


15 


2 


19 10 


Malt 


1-7 


8 


5 


1 


38 


18 


11 


2 


14 


Malt dust or coombs 


3-8 


1-8 


2-0 


2 


85 


40 


45 


4 


1 14 11 


Brewers' grains (wet) 


85 


4 


05 


1 


19 


9 


1 


2 


062 


(dried) 


3-3 


1-6 


2 


4 


74 


36 


4 


9 


142 


Molasses . 


1-5 


05 


2-5 


3 


33 


1 


56 


7 


19 1 


Meat meal 


11-5 


7 


1 


4 


257 


15 


2 


9 


3 11 


Wheat . 


1-8 


9 


6 


05 


40 


20 


13 


1 


15 2 


Barley . 


1-6 


8 


6 


05 


36 


18 


13 


1 


13 10 


Oats. 


1-7 


7 


5 


1 


45 


15 


11 


2 


13 10 


Rye. 


1-8 


9 


6 


05 


40 


20 


13 


1 


15 2 


Maize 


1-7 


6 


4 


05 


38 


13 


9 


1 


13 2 


Beans 


4-0 


1-2 


1-3 


1 


90 


27 


29 


2 


1 11 11 


Peas 


3-6 


9 


1-0 


1 


81 


20 


22 


2 


177 


Straw wheat . 


45 


2 


8 


2 


10 


4 


18 


4 


064 


barley 


5 


2 


11 


3 


11 


4 


24 


7 


7 10 


oat 


55 


2 


1-5 


4 


11 


4 


33 


9 


099 


rye 


45 


2 


9 


3 


10 


4 


20 


6 


069 


., bean . 


1-3 


3 


1-9 


1-2 


29 


7 


42 


27 


16 1 


pea 


1-4 


4 


1-0 


1-6 


31 


9 


22 


36 


13 4 


Meadow hay . 


1-5 


4 


1-6 


1-0 


34 


9 


36 


22 


16 4 


Clover hay 


2-2 


6 


1-8 


2-0 


50 


13 


40 


44 


118 


Pasture grass . 


5 


15 


6 


4 


11 


3 


13 


9 


059 



2 3 j? or references, see p. 340. 



z 2 



340 



APPENDIX II 
MANURIAL INGREDIENTS contin ned. 





Per Cent. 


Per Ton. 




Food. 


1 


o "3 


s 


S 
1 


1 


It 


1 


1 






5 


O^j 


1 


I 


|j 


|S 


1 


2 


"cs g C 'g 7*. to 






4 


2 


1-1 




fi 1 





rf 


flMfl 




Per 


Per 


Pet- 


Per 














cent. 


cent. 


cent. 


cent. 


Lb. 


Lb. 


Lb. 


Lb. 


s. /. 


Clover (green) 


55 


15 


5 


5 


13 


3 


11 


11 


058 


Vetches ( ,, ) 


55 


15 


5 


5 


13 


3 


11 


11 


058 


Lucerne ( ,, ) 


65 


15 


4 


9 


14 


3 


9 


20 


5 11 


Cabbage ( ) 


4 


15 


4 


2 


10 


3 


9 


4 


044 


Rape 


45 


15 


3 


2 


10 


3 


7 


4 


043 


Turnip tops 


35 


15 


2 


4 


8 


3 


4 


9 


033 


Turnips . 


2 


1 


3 


05 


4 


2 


7 


1 


027 


Swedes . 


25 


1 


3 


1 


5 


2 


7 


2 


2 11 


Mangels . 


2 -1 


5 


05 


4 


2 


11 


1 


035 


Carrots . 


2 -1 


3 


1 


4 


2 


7 


3 


027 


Sugar beet 


2 -1 


4 


05 


4 


2 


9 


1 


030 


Potatoes . 


;} -15 


6 


05 


7 


3 


13 


1 


046 


Cow's milk whole . 


55 


2 


15 


15 

















,, skim or 


















separated 


r> -2 


2 


15 

















Whey . . . 


15 -1 


15 


1 


~ 


~ 


~ 


~ 






1 To get approximately the equivalent amounts of ammonia, increase 
by one -fifth. 

2 To get approximately the equivalent amounts of phosphate of 
lime, multiply by 2. 

8 In calculating these manurial values the unit prices adopted by 
Hall and Voelcker (Journal of the Royal Agricultural Society, Vol. 63, 
1902, p. 108) have been employed, viz. : 

Nitrogen = 12s. (= ammonia at 9s. 10|fZ. 

Phosphoric acid = 3s. ( phosphate of lime at Is. ^d. 

Potash = 4s. 

Lime is not taken into account. 



INDEX 



ABSORPTION of salts by soils, 
103 

Acid extract, total, 121 

Acidity, estimation of, 115 

Adaptability of micro-organisms, 
149 

Adaptation of manures, 182 

Addition of plant foods, 174 

Adulteration. See Various man- 
ures. 

Aerobic bacteria, 140 

Air, atmospheric, 10 

Air of soils, 134 

Air dry soils, 96 

Aitkeii on nitragin, 148 

Albite, 22 

Albuminoids, 317 

Algse, 10, 14, 137 

Algerian phosphate, 210 

Alinite, 151 

Alkaline soils, 114 

Alluvial soils, 49 

Alluvium, 28 

Alumina in soils, 121 

Alumiiiates, 106 

Aluminium alkali-carbonate, 106 
hydrate, 59, 106, 

132 
phosphate, 133, 220 

Alta Vela phosphate, 212 

Altitude, influence of, 76 

Amandus Kahl's sieves, 234 



American Bureau of Soils, 

190 

American ashes, 278 
Amides, 154 
Amino- acids, 154 
Ammonia, absorption of by soil, 

13, 161 
fixers, 322 
,, in air, 12 

,, in rain water, 16 

,, in farmyard manure, 

317 

,, oxidation of, 156 

,, production of from 

proteids, 154 
,, retention of by soils, 

105, 257 

Ammonium carbamate, 316 
,, carbonate, 316 

chloride, 258 
,, citrate, 220 

,, nitrate, 258 

,, sulphate, 253 

Anaerobic bacteria, 140 
Analysis of plants, 4 

,, soils, chemical, 96, 

122, 191 

,, ,, mechanical, 88 

Angamos guano, 296 
Animal excreta, 306 

,, manures, 293 
Anorthite, 22, 128 



342 



INDEX 



Antiseptics, 140, 323 

Apatite, 20, 209 

Apocrenic acid, 109 

Apparent specific gravity of soils, 

56 

Appendix, 332 
Aqueous rocks, 24 
Arable soils, nitrogen in, 14 
Arenaceous rocks, 28 
Argillaceous rocks, 28 
Arrangement of particles in soil, 

51 

Artichokes, manure for, 199 
Artificial guanos, 293 
Artificial manures, 3, 202 
Ash of plants, 5, 6, 278 
Asparagin, 154 
Asparagus, 199 
Aspect, 76 

Assimilation of plant food, 124 
Assimilative capacity, 194 
Atmosphere, 10 
Augite, 21 
Available lime, 115 
Available plant food, 90, 123 
Available state, 124 



BACILLI, 139 
Bacillus mycoides, 156 

,, radicicola, 146 
Bacteria, 14, 40, 138 
Ballestas guano, 296 
Barley, ash of, 6 

,, composition of, 169 

,, manure for, 195* 

,, straw, 312 
Basalt, 30 
Basic cinder, 224 

,, phosphate, 229 
Basic slag, 224 

,, adulteration, 234 



Basic slag, application, 233 
,, artificial, 235 
,, compared with super- 
phosphate, 232 
,, composition of, 227 
,, mamirial value, 231 
,, mechanical condition, 

234 

,, production of, 224 
,, soils suitable for, 231 
solubility of, 230, 234 
Bean straw, 313 
Beans, ash of, 6 

,, composition of, 169 
,, manure for, 199, 288 
Beech leaves, 5, 304 

,, wood, 5 
Beetroot, 199 
Belgian phosphate, 211 
Bennigsen's method, 95 
Bessemer' s process, 224 
Biology of soils, 137 
Birkland and Eyde process, 270 
Black mica, 23 
Blood manures, 251 
Bone ash, 242 
,, char, 243 
,, meal, 243 
,, superphosphate, 244 
Bones, 238 

Bordeaux phosphate, 210 
Buckland on coprolites, 212 
Butyric acid in farmyard manure, 
315 



CABBAGES, manure for, 199 

Cakes as manure, 303 

Calcite, 20 

Calcium carbonate, 111 
,; chloride, 59 
,, cyanarnide, 271 



INDEX 



343 



Calcium fluoride, 20 
nitrate, 269 

,, phosphate, 207, 215. 
,, in plants, 6, 116 
Calcareous pans, 113 
,, sand, 33, 42 

soils, 48, 122 
Caliche, 260 
Canadian apatite, 209 
Capacity for heat, of soils, 78 

,, ,, water, of soils, 60 
Capillarity, 63 
Carbamide, 154 

Carbo-hydrates in farmyard man- 
ure, 315 

Carbon in humus, 111 
,, in plants, 5 
,, dioxide, absorption of, by 

plants, 15 

,, ,, in air, 15 

,, ,, in soil gases, 136 

Carnallite, 281 
Carolina phosphate, 210 
Caribbean phosphates, 212 
Carrots, manures for, 199, 285 
Castor cake as manure, 303 
Catch crops, 301 
Cauliflowers, manure for, 199 
Celery, manure for, 199 
Cellulose in farmyard manure, 

315 
Cereals, composition of, 169, 194 

,, manures for, 195 
Chalk, 37,43, 112 
Chalky soils, 48, 112, 122, 257 
Charleston phosphate, 210 
Chemical analysis of soils, 96, 

122, 191 

,, changes due to bac- 
teria, 141 

,, ,, in soil, 127 

,, constituents of soil, 2 



Chili saltpetre, 259 

Chlorides, effects of, on plants, 

284, 323 

,, in plants, 6 
,, in rain-water, 17 
Chlorophyll in algse, 137 
Citric acid extract of soils, 124 
,, ,, solvent for phosphates, 

235 
Citrate soluble phosphate in basic 

slag, 230 
Citrate soluble phosphate in dis- 

. solved bones, 245 
Citrate soluble phosphate in super- 
phosphates, 220 
Clay, coagulation of, 86 
,, composition of, 36 
,, properties of, 33 
,, soils, 42 
Classification of crops, 193 

,, ,, manures, 202 

,, ,, soils, 42 

Climate, 74 

Clover, composition of, 169 
,, effect of lime on, 117 
,, manures for, 197, 285 
Coagulable clay, 85 
Coal, nitrogen in, 253 
Coherence, 33 
Colloidal clay, 85 

hydrates, 59, 67. 85, 

106 

Colloids, 88 
Colour of soils, 78, 83 
Combined water in soils, 97 
Common salt, 59, 289 
Comparison of artificial and farm- 
yard manures, 
182 

,, ,, basic slag an 

superphosphate, 
232 



344 



INDEX 



Comparison of sulphate of am- 
monia and ni- 
trate of soda, 265 
,, ,, sulphate and muri- 

ate of potash, 
284 

Composts, 153 
Compound manures, 287 
Conductivity of soils, 81 
Congelo, 260 
Consolidation of soils, 53, 61, 72, 

81, 160 

Conservation of matter, 176 
Contraction of soils, 88 
Copperas, 199, 291 
Coprolites, 212 
Corn crops, manures for, 195 
Cow dung, 306, 326 
Craie-grise, 211 
Crenic acid, 109 
Crops, composition of, 169, 192 

classification of, 193 

effect of removing, 165 

garden, 197, 285 

,, requirements of, 192 

,, See Various crops. 
Crude potash, 278 
Crushed bones, 241 
Crust of the earth, composition 

of, 23 

Cucumbers, manures for, 199 
Cultivation, 42 
Cyanamide, 271 

Cycle of changes, 164, 168, 175 
Cystine, 154 



DALMARNOCK sewage works, 299 
Decay, 151 

Decomposition of minerals, 128 
,, organic matter, 

138, 151 



Definition of manure, 178 
Defloculation, 86 
Dehydration, 132 
Deliquescence, 59 
Deltas, formation of, 27 
Denitrification in farmyard man- 
ure, 317 

,, ,, soils, 159 

Density of soils, 55 
Denudation, 27 
De-oxidation, 11 

Destruction of organic matter, 151 
Detritus, 27 
Dew, 58 
Diffusion of gases in soil, 135 

,, salts in soil, 88 
Dissolved bones, 243 

guano, 297 
Dolomite, 37 

powder, 226 
Double superphosphates, 218 
Drain-gauges, 67 
Drainage, influence on nitrifica- 
tion, 160 

,, water, amount of, 68 

,, ,, composition of, 

98 

,, ,, nitrates in, 101 

Dried blood, 251 
Drift materials, 18 

soils, 49 
Dung, 300 

Dyer, analysis of manures, 326 
soils, 124 



EARTH, composition of crust, 24 

fine, 91, 122 

,, worms, 41 
Eight-plot test, 185 
Elements, essential in plants, 8 
Elutriation of soils, 91 



INDEX 



345 



Enzemes, 138 

Equalised guano, 297 

Erosion, 25 

Estramadurite, 210 

Evaporation, amount of, 69 

,, effect on tempera- 

ture, 80 

Excreta in farmyard manure, 306 

Exhaustion of soils, 163 

Exhaustive effects of manures, 
266 



FALLOW, 132, 165 

Farmyard manure, .'305 

,, application of, 330 
,, composition of, 324 
,, excrements in, 306 
,, fermentation of, 314 
,, fertilising ingredients, 

327 

,, litter in, 312 
,, loss in making, 318 
,, manurial value, 326 
,, mechanical effects, 327 
,, preservatives for, 322 
,, retention of water by, 
62 

Feathers, 251 

Feeding stuffs as manures, 303 

Felspars, 21, 128, 277 

Fermented bones, 241 

Ferments, formless, 138, 154 

,, nitric and nitrous, 156 

Ferns as litter, 314 
,, ,, manure, 304 

Ferric oxide in soil, 122, 131 
,, phosphate, 133, 220 

Ferrous sulphate, 199, 291, 322 

Fertilising ingredients of man- 
ures, 180 

Fertility, 163 



Fine earth, 91, 122 
Fish guano, 247 
Fixation of free nitrogen, 142 
,, by non - leguminous 

plants, 151 
Floculation, 86 
Florida phosphate, 209 
Flowers, manures for, 202, 288 
Fluor-apatite, 20 
Fluorides, 20 
Fluorine in plants, 6 
Foods, manurial value of, 338 

,, proportion retained by 

animals, 167, 308 
Formation of soils, 40 
Formations, geological, 42 
French gardeners, 199, 200 

,, phosphates, 210 
Freybentos meatmeal, 246 
Frost, effects of, 25 
Fruit trees, manures for, 210, 285, 

288 

Functions of manures, 180 
Fungi, 137 



GAS lime, 119 
liquor, 253 

Garden crops, manures for, 197, 
242, 246, 251, 253, 285 

Geic acid, 109 

Gelatinous hydrates, 59, 67, 85, 
106 

General manures, 203, 292 

Geological classification of rocks, 
29,42 

German phosphates, 210 

German salts, 279 

Glaciation, 25 

Glycocine, 154 

Grain and grass crops, com- 
position of, 169, 194 



346 



INDEX 



Grain and grass crops, manures 

for, 195, 287, 288 
Grandeau's method, 111 
Granite, 29, 30 
Gravel, 32, 51 
Green manures, 301 
Greenland guano, 296 
Ground lime, 119 
Guano, application of, 298 
artificial, 293 

,, composition of, 296 

,, imports of, 295 

,, mammal value of, 297 

,, nitrogenous, 295 

,, origin, 294 

,, Peruvian, 295 

,, phosphutic, 214, 295 
Gypsum as manure, 199, 291 

,, ,, preservative, 322 

,, mineral, 20 



HEMATITE, 19 

Hair as manure, 251 
Hay, composition of, 169 
Heat, conduction of, 81 
,, capacity of soils for, 78 
latent, 80 
,, production of, 82, 198 
,, radiation of, 80 
,, sources of, 74 
,, specific, 7s 
Heavy manures, 203 
Heavy soils, 35, 48 
Heinrich's experiments, 62 
Hellriegel on fixation of nitrogen, 

143 
,, ,, optimum amount of 

water, 62 

,, ,, transpiration, 73 
Hendrick on basic slag, 231 
,, ,, seaweed, 303 



Highland Society's unit values, 

333 

Hilgard on floculation, 87 
Hilgenstock on slag phosphates, 

229 

Hiltner's culture, 149, 150 
Herring guano, 247 
Hoof meal, 250 
Hops, manure for, 242, 251 
Hornblende, 21 
Horn meal, 250 
Horse dung, 306, 326 
Hughes on composition of ferns, 

304 

Humates, 110, 316 
Humic acid, 85, 87, 109 
Hurnin, 109 
Humous soils, 42 
Humus, 108 

Hydrates, deliquescent, 59 
Hydrochloric acid extract of soils, 

120 

Hydrogen in plants, 5 
Hydrolysis, 154 
Hygroscopy, 58 



ICELAND guano, 296 

Ichaboe guano, 296 

Igneous rocks, 24 

Imports of bones, 2.'>!) 
,, ,, guano, 295 
,, ,, nitrate of soda, 262 
,, ,, potash salts, 282 
,, ,, raw phosphates, 'JOS 

Impurities in air, 15 

Inoculation of soils, 146, 149 

Insoluble mineral residue of soils, 
120 

Internal surface of soils, 54, 108 

Interspace in soils, 51, 89 

Iodine in Chili- saltpetre, 261 



INDEX 



347 



Iron in plants, 6 
,, ,, soils, 171) 
,, hydrates of, 59, 132 
sulphate of, 199, 291 

Irrigation, 29!) 



KAHL'S sieves, 234 
Kaiiiite, 282, 323 
Kaolin, 22, 107, 128 
Kelp, .27S 
Kieserite, 282 
King 011 transpiration, 73 
Kostytcheff on composition of 
humus, 110 



LACTIC acid in farmyard manure, 

315 

Lahn phosphate, 210 
Lasting manures, 2(><S 
Latent heat of water, 80 
Latitude, influence of, 74 
Lawes dissolved phosphates, 206 
,, and Gilbert on transpira- 
tion, 73 
Lawes and Gilbert on loss of 

lime, 100 
Lay, 166 
Leaf mould, 304 
Leather as manure, 250 
Leaves as litter, 314 
Leeks, manure for, 199 
Leguminosee, 14 
Leguminous crops, composition of 

169, 194 

,, ,, fixation of 

nitrogen by, 
142 
,, ,, manures for, 

197 
Lettuces, manure for, 199 



Leucine, 154 

Leucite, 21 

Lichens, 40 

Liebig dissolved bones, 207 

Light manures, 203 

,, soils, 32, 48 
Lime, action of, in soils, 115 

,, application of, 118 

,, available, 115 

,, effect of, on clovers, 117 

. > ,, moss, 117 

> ,, ,, manures, 116 

,, ,, ,, nitrification, 

115, 158 

,, loss of, 100 

,, properties of , 113 

,, Seeaho Calcium compounds. 
Lime-felspar, 22, 128 
Lime-nitrogen, 270 
Limestones, 37, 112 
Limonite, 19 

Linseed cake as manure, 303 
Liquid manure, 300 
Lithium in plants, 5 
Litter, kinds of, 312 

,, quantity of, 314 
Loamy soils, 42 
Lot phosphate, 210 
Lucerne hay, 310 
Lupines as green manure, 301 



MAERCKER and Schneidewind's 

experiments, 321 
Magnesia in plants, 6 
,, ,, soils, 180 
Magnesian limestone, 37 
Magnesium salts as manure, 291, 

322 

Magnetite, 19 
Manganese in plants, 5 
soils, 132 



348 



INDEX 



Mangolds, composition of, 169, 

194 

manures for, 196, 288 
Manures, adaptation of, 182 
animal, 204, 292 
,, artificial, 3, 202 
,, classification of, 202 
,, compound, 287 
,, constituents of, 179 
,, definition of, v 178 
,, for crops. See Crops. 
,, functions of, 180 
,, general, 203, 292 
,, mineral, 204 
,, miscellaneous, 205, 289 
,, nitrogenous, 249 
,, phosphatic, 206 
,, phospho-nitrogenous, 

237 

potash, 276 
,, special, 203 
,, valuation of, 333 
, , vegetable manures, 204, 

292 

,, See also Various man- 
ures. 
Manurial requirements of crops, 

192 
Manurial requirements of soils, 

182, 185 
Manurial elements, ISO 

,, value of foods, 338 
Manuring, principles of, 17S 
Marl, 35, 112 
Marrows, manure for, 199 
Mass of soil, 55 
Mayer's experiments, 61 
Maxima and minima, law of, 183 
Meadow hay, composition of, 169 

,, ,, manure for, 195 

Mechanical analysis of soils, 88 
, , constituents of soils, 2 



Mechanical condition of hasic 

slag, 234 

Melons, manure for, 199 
Metamorphic rocks, 29 
Methane in farmyard manure, 315 
Mica, 21, 277 
Micaceous sand, 21, 277 
Microbes, 148 
Micro cci, 139 
Micrometer, 95 
Micro-organisms in air, 15 

,, ,, soil, 137 

Microscopic examination of basic 

slag, 234 
Microscopic examination of soils, 

94 
Minerals, 19 

,, composition of, 23 
,, decomposition of, 128 
Mineral manures, 204 

,, matter in plants, 4 
Miscellaneous manures, 2S9 

,, properties of soils, 

83 

Mixed manures, 287 
Moisture in soils, 57 
Moles, 41 

Moorband pan, 115 
Moore's culture, 149, 150 
Moss in pastures, 117 

,, litter, 313 
Moulds, 138 
Mulching, 72 
Mulder on humus, 109 
Muriate of ammonia, 258 

of potash, 283 

Mustard as green manure, 301 
Mycoides, bacillus, 156 



NASSAU phosphate, 210 
Native phosphates 206 



INDEX 



349 



Natrolite, 22 
Natural manures, 202 

,, productiveness, 163 
Nepheline, 22 
Nitragin, 148 
Nitrates, formation of, 103, 152 

,, in drainage, 101 
Nitrate of lime, 269 

potash, 201, 269 
,, ,, soda, adulteration of, 

263 
,, ,, ,, application of, 

264 

,, ,, ,, compared with 
sulph. of am., 
265 

,, ,, ,, deposits of, 259 
,, ,, ,, extraction of, 

260 

,, ,, ,, exhaustive 

effects of, 266 

,, ,, ,, manurial value 

of, 263 

Nitric acid in air, 12 
,, ,, in rain, 16 
,, ,, in soil, 103, 156 
,, ferments, 156 
Nitrification, 153 
Nitrites, 159 
Nitrogen, atmospheric, 12 

, , in animal excreta, 309 
,, ,, crops, 169, 193 
,, ,, humus, 110 
,, ,, manures, 180. See 
also Various manures. 
,, ,, plants, 5 
,, ,, rain, 16 

,, soils, 14, 161 
,, fixation of, 144 *v^^j 

loss of, 101 

,, retained by animals, 
167, 308 



Nitrogenous guanos, 295 

,, manures, 249 

Nitrolim, 271 
Nitrous acid, 153, 161 
Nitrous ferment, 156 
Nodules on roots, 143 
Number of particles in soil, 51 



OATS, composition of, 169 

,, manures for, 195 
Oat straw, 312 
Odour of soils, 84 
Olivine, 21 

Onions, manure for, 199, 285 
Oolitic limestone, 37 
Optimum, amount of water, 62 
Organic matter, destruction of, 



,, ,, estimation of , ttl 

,, ,, formation of, 40 

,, in air, 10" 

,, ,, in plants, "> 

,, ,, in soils, 108 

,, rocks, 28 

Organisms in soil, 137 

Origin of soils, 18 

Orthoclase, 21, 129, 277 

Osborne's method, 92 

Ossein, 239 

Oxidation, 11 

,, in soil, 131 

Oxygen, atmospheric, 11 

,, absorption of, by plants, 

5 

,, evolved by plants, 15 
,, in soil gases, 135 



PAN formation, 33, 113, 115 
Parasitic fungi, 138, 139 
Parsley, manure for, 199 



350 



INDEX 



Parsnips, manure for, 199 

Particles of soil, 50 

Pasteur's experiments, 141 

Pasture soils, nitrogen in, 14 

Patagonian guano, 296 

Pearl ash, 27 S 

Peas, manures for, 199, 288 

Pea straw, 313 

Peat, 37, 324 

Peat litter, 313 

Perchlorates in nitrate of soda, 

263 

Percolation, 66, 68 
Peruvian guano, 295 
Phosphate, aluminium, 133, 220 
basic, 229 

,, dicalcic, 215 

ferric, 133, 220 

,, monocalcic, 215 

,, tetracalcic, 229 

,, tricalcic, 215 

,, reverted, 219 

Wiborgh, 235 
Phosphates, native, 206 

,, in soils, 133 

Phosphatic guanos, 214, 295 

,, manures, 206 

Phospho-nitrogenous manures, 

237 
Phosphoric acid, 59, 216 

,, in crops, 169 

,, ,, manures, 180. See 

also Various 
manures. 

,, ,, plants, 6 

,, ,, soils, 122 

,, retention of, 105 

Phosphorite, 209 
Physical constituents of soils, 2 
,, properties of soils, 50 
Pig-iron, 224 
Pigs' dung, 306 



Pipe-clay, 59 
Plant food, 1, 3, 5, 7 

,, ,, amount required by 

crops, 168 

,, ,, available, 123 
,, ,, retained by animals, 

167, 309 

Plants, analysis of, 4 
,, ash of, 6 

,, essential constituents, 8 
Plastic clay, 33, 87 
Plutonic rocks, 2!) 
Polyhalite, 280, 2S2 
Portugese phosphate, 210 
Potash in drainage, 99 

manures, ISO, 283. ,S'ee 
also Various manures. 
,, ,, plants, 6 
,, ,, rocks, 30 
,, soil, 122, 285 
,, liberation of, 130 

minerals, 277 
,, retention of, 105 
salts, 279 

Potash manures, crops which 

require, 285 

,, ,, soils suitable 

for, 285 
Potashes, 27s 
Potassium cyanide, 13 
nitrate, 269 

Potatoes, composition of, 169, 194 
,, manures for, 196, 199, 
288 

Precipitated phosphate, 235 
Principles of manuring, 178 
Produce, loss by sale of, 171 
Productiveness, natural, 163 
Proteids, conversion to ammonia, 

154 

Pumpkins, manure for, 199 
Putrefaction, 151 



INDEX 



351 



Pyrites, 20, 220 



QUARTZ, 20 
Quicklime, 118 



RADIATION of heat from, soil, 80 
Radicicola bacillus, 146 
Radishes, manure for, 199 
Rain, amount of, 58, 68 
composition of, 16 
gauges, 67 
Rape for green manure, 301 

,, cake as manure, 303 
Raw bones, 241 
Reactions, various, in soil, 132 
Rectified guano, 297 
Red clover, 169 
Redonda phosphate, 212 
Reduced phosphate, 219 
Reduction in soils, 10, 131 
Refuse cakes as manure, 204, 303 
Restitution, 167 
Restoration, 180, 181 
Retention of plant food in soil, 

100, 105 

,, ,, water in soil, 60, 90 

Reverted phosphate, 219 
Rocks, 24 
Root crops, composition of, 169, 

194 

,, ,, manures for, 195 
Roots, assimilative power, 124, 

194 

,, nodules on, 143 
Rothamsted experiments 

,, crops, yield from un- 

man ured plots, 172 
drainage, composition 

of, 98 
nitrogen in, 102 



Rothamsted experiments manures, 
continuous applica- 
tion of, 266 
manures, joint action 

of, 184 

rainfall, percolation 
and evaporation, 
68, 69 

,, rain water, com- 
position of, 17 
,, soils, accumulation of, 
nitrogen in, 14 
,, ,, inoculation of, 

149 

,, ,, volume weight 

of, 57 

SALE of produce, loss by, 171 
Salt, common, 289 
Sampling soils, 89 
Sand, 31 

,, cultures, 7, 143 

,, stones, 33, 113 
sandy soils, 42 
saprophytic fungi, 138, 139 
sawdust as litter, 314 
Schlcesing on humic acid, 87, 110 
,, ,, retention of water, 

61 

Schone's method, 92 
Schonite, 281 
Sea-sand, 31 
Seat of fixation, 146 
Seaweed, 302 
Sedimentary rocks, 28 
Seeds, 7, 143, 170, 198 
Sewage, 298 
Seychelles guano, 296 
Shales, 35 

Shallots, manure for, 199 
Sheep's dung, 307 
Shells, 112 



352 



INDEX 



Shell sand, 32 

Shoddy, 251 

Shrinkage of soils, 88 

Sieve analysis of basic slag, 234 

soils, 50, 91 
Silica in minerals, 20 

,, plants, 6, 169, 195 

soils, 59, 85, 129, 132 
Silicates, 19, 107, 128, 195, 205 
Silt, 28, 50 

Size of particles, 31, 34, 50, 91 
Slag. See Basic slag. 
Slaked lime, 118 
Slate, 35 

Sludge manures, 299 
Sodium salts in drainage, 98 

plants, 5, 6, 169 
,, ,, soils, 122. See also 

Nitrate of soda. 
Soils, air of, 134 

,, absorption of gases by, 13, 

59, 161 
,, absorption of salts by, 100, 

105 

,, acidity of, 115 
,, alkaline, 114 
,, analysis of, chemical, 96, 

122, 191 
,, ,, ,, mechanical, 

88 

,, biology of, 137 
,, chemistry of, 96 
,, classification of, 42 
,, fertility of, 163 
,, formation of, 40 
,, manurial requirements of, 

182 
,, miscellaneous properties, 

of, 83 

,, nitrogen of, 161 
,, organic matter of, 2, 40, 
108, 111, 151 



Soils, origin of, 18 

,, physical properties of, 50 
, , sampling of, 89 
,, sour, 113 
,, temperature of, 74, 82 
., water of, 57, 96 
Solar heat, 74 
Solutionary rocks, 28 
Somme phosphate, 211 
Soot, 258 
Sour soils, 113 

Sources of plant constituents, 7 
South Carolina phosphates, 210 
Spanish phosphorite, 210 
Special manures, 203 
Specific gravity of soils, 56 

,, heat of soils, 78 
Sphagnum, 38 
Spinach, manure for, 199 
Stalactites, 112 
Stassfurth salts, 279 
Steamed bones, 242 
Steel, Bessemer's process for, 224 
Sterilisation, 140 
Stiff land, 48 
Stilbite, 22 

Stockbride on humus, 109 
Storer on farmyard manure, 326 
Stratification, 29 
Straw as litter, 312 
Strawberries, manure for, 200 
Sulphate of ammonia, 253 
,, adulteration of, 255 
,, application of, 257 
,, compared with nitrate 

of soda, 265 

,, composition of, 255 
,, production of, 253 
,, manurial value of, 256 
of iron, 199, 291 
of potash, 281, 283 
Sulphates in rain-water, 17, 179 



INDEX 



353 



Sulphides, effect on plants, 235, 

323 

Sulphur in plants, 5 
Superphosphates, 214 

,, application of, 

222 

,, bone, 244 

,, compared with 

basic slag, 
232 
,, composition of, 

217 

double, 218 
,, manufacture of , 

216 
,, manurial value 

of, 221 

Surface tension, 60 
Swedes, composition of, 169 

,, manure for, 196 
Symbiosis, 14, 138, 14(5 
Sylvine, 281 
Sylvinite, 281 

TALC, 23 

Tannery refuse as litter, 314 

Temperature of soils, 74, 82 

Tenacity of soils, 85 

Tetracalcic phosphate, 230 

Texture of soils, 50 

Thallophytse, 137 

Tilth, 87 

Thomas & Gilchrist process, 226 

Thomas' phosphate. t$ee Basic 

slag. 

Tomatoes, manure for, 199 
Total acid extract of soils, 121 
Transpiration, 73 
Tricalcic phosphate, 207, 216 
True specific gravity of soils, 56 
Truffont's manure for straw- 
berries, 201 
S.M. 



Turnips, composition of, 169, 194 
manure for, 196, 199, 287 
Tyrosine, 154 



ULMIC acid, 109 

Ulmin, 109 

Unit values of manures, 335 

Unmanured plots, yield of, 173 

Unoccupied space in soils, 54 

Unstratified rocks, 29 

Urea, 154 

Uric acid, 294 

Urine of farm animals, 308 

Uruguay guano, 296 

Uruguay meat meals, 246 



VALUATION of manures, 332 

Vegetable manures, 292, 301 
,, soils, 48 

Venezuela guano, 296 

Vine manure, 287 

Vetches, 143, 301 

Voelcker on composition of apa- 
tite, 209 

Voelcker on composition of cal- 
cium cyanamide, 273 

Voelcker on composition of drain- 
age water, 98 

Voelcker on composition of farm- 
yard manure, 325 

Volatile matter of plants, 4 

Volcanic rocks, 29 

Volume weight of soils, 56 



WARINGTON on composition of 

rain water, 17 
Warington on composition of 

crops, 169 
! Warington on composition of 



manure, 308, 325 



A A 



354 



INDEX 



Water in soils, 57, 96 

,, cultures, 8 

films, 60 

,, vapour, 14 
Way's experiments, 105 
Weathering of rocks, '25 
Weeds, 201, 279, 331 
West Indian phosphates, 212 
Wheat, composition of, 169 
,, manure for, 195 
,, straw, 312 
White mica, 23 
Whitney on absorption, 107 



Wiborgh phosphate, 235 
Wind-blown sands, 27 
Winogradsky on nitrification, 156 
Wire-basket method, 190 
Woburn experiments, 321 
Wollny on transpiration, 73 
Wood ashes, 277 
Woollen rags, 251 



YEASTS, 138 



ZOOLITES, 22, 107 







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